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.

NEWS

@@ -3,6 +3,20 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-09-11:
+
+- The 'command' regular builtin utility (which runs a simple command, removing
+  special properties) has been made fully POSIX compliant.
+  1. The 'command' name can now result from an expansion (fixing BUG_CMDEXPAN),
+     e.g. 'c=command; "$c" ls' and 'set -- command ls; "$@"' now work.
+  2. If and only if the POSIX mode (the new -o posix shell option) is active,
+     then the 'command' utility now disables not only "special" but also
+     "declaration" properties of builtin commands that it invokes, meaning:
+     a. arguments that start with a variable name followed by '=' are
+	always treated as regular words subject to normal shell syntax;
+     b. 'command' can now stop the shell from exiting if a command that it
+	invokes tries to modify a readonly variable (fixing BUG_CMDSPEXIT).
+
 2020-09-09:
 
 - Fixed BUG_LOOPRET2 and related bugs. The 'exit' and 'return' commands without