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/bltins/misc.c

@@ -60,6 +60,9 @@ struct login
 	char    *arg0;
 };
 
+/*
+ * 'exec' special builtin and 'redirect' builtin
+ */
 int    b_exec(int argc,char *argv[], Shbltin_t *context)
 {
 	struct login logdata;
@@ -68,7 +71,7 @@ int    b_exec(int argc,char *argv[], Shbltin_t *context)
 	logdata.arg0 = 0;
 	logdata.sh = context->shp;
         logdata.sh->st.ioset = 0;
-	while (n = optget(argv, sh_optexec)) switch (n)
+	while (n = optget(argv, *argv[0]=='r' ? sh_optredirect : sh_optexec)) switch (n)
 	{
 	    case 'a':
 		logdata.arg0 = opt_info.arg;
@@ -84,9 +87,11 @@ int    b_exec(int argc,char *argv[], Shbltin_t *context)
 		errormsg(SH_DICT,ERROR_usage(0), "%s", opt_info.arg);
 		return(2);
 	}
-	argv += opt_info.index;
 	if(error_info.errors)
 		errormsg(SH_DICT,ERROR_usage(2),"%s",optusage((char*)0));
+	if(*argv[0]=='r' && argv[opt_info.index])  /* 'redirect' supports no args */
+		errormsg(SH_DICT,ERROR_exit(2),e_badsyntax);
+	argv += opt_info.index;
 	if(*argv)
                 B_login(0,argv,(Shbltin_t*)&logdata);
 	return(0);