Fix 'command' expansion bug and POSIX compliance

The 'command' name can now result from an expansion, e.g.:
	c=command; "$c" ls
	set -- command ls; "$@"
both work now. This fixes BUG_CMDEXPAN.

If -o posix is on, 'command' now disables not only the "special"
but also the "declaration" properties of builtin commands that it
invokes. This is because POSIX specifies 'command' as a simple
regular builtin, and any command name following 'command' is just
an argument to the 'command' command, so there is nothing that
allows any further arguments (such as assignment-arguments) to be
treated specially by the parser. So, if and only if -o posix is on:
a. Arguments that start with a variable name followed by '=' are
   always treated as regular words subject to normal shell syntax.
b. Since assignment-arguments are not processed as assignments
   before the command itself, 'command' can now stop the shell from
   exiting (as required by the standard) if a command that it
   invokes (such as 'export') tries to modify a readonly variable.
   This fixes BUG_CMDSPEXIT.

Most of 'command' is integrated in the parser and parse tree
executer, so that is where it needed fixing.

src/cmd/ksh93/sh/parse.c: simple():
- If the posix option is on, do not skip past SYSCOMMAND so that
  any declaration builtin commands that are arguments to 'command'
  are not detected and thus not treated specially at parsetime.

src/cmd/ksh93/sh/xec.c: sh_exec():
- When detecting SYSCOMMAND in order to skip past it, not only
  compare the Namval_t pointer 'np' to SYSCOMMAND, but also handle
  the case where that pointer is NULL, as when the command name
  results from an expansion. In that case, search the function tree
  shp->fun_tree for the name and see if that yields the SYSCOMMAND
  pointer. fun_tree is initialised with a dtview to bltin_tree, so
  searching fun_tree instead allows for overriding 'command' with a
  shell function (which the POSIX standard requires us to allow).

src/cmd/ksh93/sh.1,
src/cmd/ksh93/data/builtins.c:
- Update documentation to match these changes.
- Various related edits and improvements.

src/cmd/ksh93/tests/builtins.sh:
- Check that 'command' works if resulting from an expansion.
- Check that 'command' can be overridden by a shell function.

src/cmd/ksh93/sh.1

@@ -3691,7 +3691,7 @@ are equivalent (as far as the above execution of
 .I cmd\^
 is concerned except for special built-in commands listed below \-
 those that are
-preceded with a dagger).
+marked with \fB\(dg\fR).
 .PP
 If the obsolete
 .B \-k
@@ -5495,10 +5495,11 @@ Commands that are preceded by a \(dd symbol below are
 Any following words
 that are in the format of a variable assignment
 are expanded with the same rules as a variable assignment.
-This means that
-tilde substitution is performed after the
+This means that tilde substitution is performed after the
 .B =
-sign and field splitting and file name generation are not
+sign, array assignments of the form
+\f2varname\^\fP\f3=(\fP\f2assign_list\^\fP\f3)\fP
+are supported, and field splitting and file name generation are not
 performed.
 .PD
 .TP
@@ -5650,7 +5651,7 @@ via the
 file will associate with the pathname of the directory containing the
 .B .paths
 file.
-.P
+.IP
 The ISO C/C++ prototype is
 \f3b_\fP\f2mycommand\fP\f3(int\fP \f2argc\fP, \f3char *\fP\f2argv\fP\f3[]\fP, \f3void *\fP\f2context\fP\f3)\fP
 for the builtin command
@@ -5663,7 +5664,7 @@ elements and context is an optional pointer to a
 .B Shell_t
 structure as described in
 .BR <ast/shell.h> .
-.sp .5
+.IP
 Special built-ins cannot be bound to a pathname or deleted.
 The
 .B \-d
@@ -5770,50 +5771,6 @@ command may not be executed by
 .if \nZ=2 .B rksh93\^.
 .TP
 \f3command\fP \*(OK \f3\-pvxV\fP \*(CK \f2name\^\fP \*(OK \f2arg\^\fP .\|.\|. \*(CK
-Without the
-.B \-v
-or
-.B \-V
-options,
-.B command
-executes
-.I name\^
-with the arguments given by
-.IR arg .
-The
-.B \-p
-option causes
-the operating system's standard utilities path
-(as output by \f3getconf PATH\fP) to be searched
-rather than the one defined by the value of
-.SM
-.BR PATH .
-Functions and aliases will not be searched for when finding
-.IR name .
-In addition, if
-.I name\^
-refers to a special built-in,
-none of the special properties associated with the leading
-daggers will be honored.
-(For example, using
-.B "command set -o"
-.I "option-name"
-prevents a script from terminating when an invalid
-option name is given.)
-With the
-.B \-x
-option,
-if command execution would result in a failure because
-there are too many arguments, errno
-.SM
-.BR E2BIG ,
-the shell will invoke command
-.I name\^
-multiple times with a subset of the arguments on each invocation.
-Arguments that occur prior to the first word that
-expands to multiple arguments and after the last word
-that expands to multiple arguments will be passed on each invocation.
-The exit status will be the maximum invocation exit status.
 With the
 .B \-v
 option,
@@ -5827,6 +5784,63 @@ option causes
 .B command
 to act like
 .BR "whence \-v" .
+.IP
+Without the
+.B \-v
+or
+.B \-V
+options,
+.B command
+executes
+.I name\^
+with the arguments given by
+.IR arg .
+Functions and aliases will not be searched for when finding
+.IR name .
+If
+.I name\^
+refers to a special built-in,
+as marked with \fB\(dg\fR in this manual,
+.B command
+disables the special properties described above for that mark,
+executing the command as a regular built-in.
+(For example, using
+.B "command set \-o"
+.I "option-name"
+prevents a script from terminating when an invalid
+option name is given.)
+If
+.I name\^
+refers to a declaration built-in,
+as marked with \fB\(dd\fR in this manual,
+and the \fBposix\fR shell option is on,
+then the declaration properties are removed so that arguments containing
+\fB=\fR are not treated specially.
+.IP
+The
+.B \-p
+option causes
+the operating system's standard utilities path
+(as output by \f3getconf PATH\fP) to be searched
+rather than the one defined by the value of
+.SM
+.BR PATH .
+.IP
+The
+.B \-x
+option allows executing non-built-in commands with argument lists exceeding
+limitations imposed by the operating system. This functionality is similar to
+.BR xargs (1)
+but is easier to use.
+If a command cannot ordinarily be executed because there are too many
+arguments, the shell will invoke the indicated command multiple times
+with a subset of the arguments on each invocation.
+Any arguments (such as command options) that come before the first word
+that expands to multiple arguments, as well as any that follow the last
+word that expands to multiple arguments, are considered static arguments
+and are repeated for each invocation. When all invocations are completed,
+.B "command \-x"
+exits with the status of the invocation that had the highest exit status.
 .TP
 \(dd \f3compound\fP \f2vname\fP\*(OK\f3=\fP\f2value\^\fP\*(CK .\|.\|.
 Causes each
@@ -7044,6 +7058,7 @@ is on by default if ksh is invoked as \fBsh\fR. It
 disables passing exported variables' attributes (such as integer or readonly) to a new ksh process through the environment,
 causes file descriptors > 2 to be left open when invoking another program,
 makes the \fB<>\fR redirection operator default to standard input,
+causes the \fBcommand\fR utility to disable declaration command properties of any declaration commands it invokes,
 disables a hack that makes \fBtest -t\fR (\fB[ -t ]\fR) equivalent to \fBtest -t 1\fR (\fB[ -t 1 ]\fR),
 enables octal numbers in \fBlet\fR shell arithmetic (see \fBletoctal\fR), and
 disables the \fB&>\fR redirection shorthand.