Make 'source' a regular built-in

The 'source' alias is now converted into a regular built-in command
so that 'unalias -a' does not remove it, and something like
	cmd=source; $cmd name args
will now work.

This is part of the project to replace default aliases that define
essential commands by proper builtins that act identically (except
you now get the actual command's name in any error/usage messages).

src/cmd/ksh93/data/aliases.c:
- Remove 'source' default alias.

src/cmd/ksh93/data/builtins.c,
src/cmd/ksh93/include/builtins.h:
- Define 'source' regular builtin with extra parser ID "SYSSOURCE".
  Same definition as '.', minus the BLT_SPC flag indicating a
  special builtin. This preserves the behaviour of 'command .'.
- Update sh_optdot[] to include info for 'source --man'.
  (Note that \f?\f expands to the current command name.
  This allows several commands to share a single --man page.)

src/cmd/ksh93/sh/parse.c:
- In the two places that SYSDOT is checked for, also check for
  SYSSOURCE, making sure the two commands are parsed identically.

src/cmd/ksh93/sh.1:
- Remove 'source' default alias.
- Document 'source' regular builtin.

src/cmd/ksh93/include/builtins.h

@@ -47,9 +47,10 @@
 #define SYSLET		(shgd->bltin_cmds+12)	/* let */
 #define SYSEXPORT	(shgd->bltin_cmds+13)	/* export */
 #define SYSDOT		(shgd->bltin_cmds+14)	/* . */
-#define SYSRETURN	(shgd->bltin_cmds+15)	/* return */
+#define SYSSOURCE	(shgd->bltin_cmds+15)	/* source */
+#define SYSRETURN	(shgd->bltin_cmds+16)	/* return */
 #if SHOPT_BASH
-#   define SYSLOCAL	(shgd->bltin_cmds+16)	/* local */
+#   define SYSLOCAL	(shgd->bltin_cmds+17)	/* local */
 #else
 #   define SYSLOCAL	0
 #endif

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-06-14"
+#define SH_RELEASE	"93u+m 2020-06-15"