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.

NEWS

@@ -10,6 +10,13 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   'alias' or 'unalias', overriding the commands by the same names. In
   technical terms, they are now regular builtins, not special builtins.
 
+- The redirect='command exec' alias has been converted 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'.
+
 2020-06-10:
 
 - The 'hash' utility is now a regular builtin instead of an alias to