Make 'redirect' a regular builtin instead of an alias of 'exec'

This commit converts the redirect='command exec' alias to a regular
'redirect' builtin command that only accepts I/O redirections, which
persist as in 'exec'. This means that:
* 'unlias -a' no longer removes the 'redirect' command;
* users no longer accidentally get logged out of their shells if
  they type something intuitive but wrong, like 'redirect ls >file'.

This should not introduce any legitimate change in behaviour. If
someone did accidentally pass non-redirection arguments to
'redirect', unexpected behaviour would occur; this now produces
an 'incorrect syntax' error.

src/cmd/ksh93/bltins/misc.c: b_exec():
- Recognise 'redirect' when parsing options.
- If invoked as 'redirect', produce error if there are arguments.

src/cmd/ksh93/data/aliases.c:
- Remove redirect='command exec' alias.

src/cmd/ksh93/data/builtins.c:
- Update/improve comments re ordering.
- Add 'redirect' builtin entry.
- sh_optexec[]: Abbreviate redirection-related documentation;
  refer to redirect(1) instead.
- sh_optredirect[]: Add documentation.

src/cmd/ksh93/include/builtins.h:
- Add SYSREDIR parser ID, renumbering those following it.
- Improve comments.
- Add extern sh_optredirect[].

src/cmd/ksh93/sh.1:
- exec: Abbreviate redirection-related documentation; refer to
  'redirect' instead.
- redirect: Add documentation.

src/cmd/ksh93/sh/xec.c:
- Recognise SYSREDIR parser ID in addition to SYSEXEC when
  determining whether to make redirections persistent.

src/cmd/ksh93/tests/io.sh:
- To regress-test the new builtin, change most 'command exec' uses
  to 'redirect'.
- Add tests verifying the exit behaviour of 'exec', 'command exec',
  'redirect' on redirections.

src/cmd/ksh93/sh.1

@@ -804,8 +804,6 @@ but can be unset or redefined:
 .TP
 .B "r=\(fmhist \-s\(fm"
 .TP
-.B "redirect=\(fmcommand exec\(fm"
-.TP
 .B "source=\(fmcommand \s+2.\s-2\(fm"
 .TP
 .B "stop=\(fmkill \-s \s-1STOP\s+1\(fm"
@@ -5802,10 +5800,11 @@ In addition, if
 refers to a special built-in,
 none of the special properties associated with the leading
 daggers will be honored.
-(For example, the predefined alias
-.B "redirect=\(fmcommand exec\(fm"
+(For example, using
+.B "command set -o"
+.I "option-name"
 prevents a script from terminating when an invalid
-redirection is given.)
+option name is given.)
 With the
 .B \-x
 option,
@@ -5916,18 +5915,11 @@ rather than the first
 to become
 .B argv[0]
 for the new process.
-Input/output arguments may appear and
-affect the current process.
 If
 .I arg\^
-is not given,
-the effect of this command is to
-modify file descriptors
-as prescribed by the input/output redirection list.
-In this case,
-any file descriptor numbers greater than 2 that are
-opened with this mechanism are closed when invoking
-another program.
+is not given and only I/O redirection arguments are given,
+then this command persistently modifies file descriptors as in
+.BR redirect.
 .TP
 \(dg \f3exit\fP \*(OK \f2n\^\fP \*(CK
 Causes the shell to exit
@@ -6770,6 +6762,23 @@ by subsequent assignment.
 When defining a type, if the value of a readonly sub-variable is not defined
 the value is required when creating each instance.
 .TP
+\f3redirect\fP
+This command only accepts input/output redirection arguments.
+It can open and close files and modify file descriptors from
+.B 0
+to
+.B 9
+as specified by the input/output redirection list (see the
+.I Input/Output\^
+section above),
+with the difference that the effect persists past the execution of the
+.B redirect
+command.
+Any file descriptor numbers greater than
+.B 2
+that are opened with this mechanism are closed when invoking another program,
+unless explicitly redirected to themselves as part of that invocation.
+.TP
 \(dg \f3return\fP \*(OK \f2n\^\fP \*(CK
 Causes a shell
 .I function