Fix 'command' prefix in enum type def pre-parsing (re: 1dc18346)

Symptom:

$ ksh -c 'command enum -i P_t=(a b); P_t -A v=([f]=b); typeset -p v'
ksh: syntax error at line 1: `(' unexpected

Expected: no syntax error, and output of 'P_t -A v=([f]=b)'.

src/cmd/ksh93/sh/parse.c: check_typedef():
- For enum, skip over any possible 'command' prefixes before
  pre-parsing options with optget (or, technically, skip anything
  else that might come before 'enum', though I don't think anything
  else is possible).
- The sh_addbuiltin() call at the end to pre-add the builtin
  obtained the node pointer to the built-in and the node flags from
  the parser tree. This did not work if a 'command' prefix was
  present. However, we don't actually need this. For parsing
  purposes, the BLT_DCL flag for a declaration built-in is
  sufficient; this is what gets the parser to accept
  assignment-arguments including parentheses. So just apply that.
  In addition, let's point it to an actual dummy built-in, 'true'
  (SYSTRUE), so that if a user does run something like 'if false;
  then enum Foo_t=(...); fi', the leaked Foo_t dummy at least won't
  do anything (not even crash).

src/cmd/ksh93/sh/parse.c

@@ -222,6 +222,9 @@ static void check_typedef(struct comnod *tp, char intypeset)
 		char **argv = dp->dolval + ARG_SPARE;
 		if(intypeset==2)
 		{
+			/* Skip over possible 'command' prefix(es) */
+			while(*argv && strcmp(*argv, SYSENUM->nvname))
+				argv++;
 			/* Skip over 'enum' options */
 			opt_info.index = 0;
 			while(optget(argv, sh_optenum))
@@ -245,11 +248,7 @@ static void check_typedef(struct comnod *tp, char intypeset)
 		}
 	}
 	if(cp)
-	{
-		Namval_t	*mp=(Namval_t*)tp->comnamp ,*bp;
-		bp = sh_addbuiltin(cp, (Shbltin_f)mp->nvalue.bfp, (void*)0);
-		nv_onattr(bp,nv_isattr(mp,NV_PUBLIC));
-	}
+		nv_onattr(sh_addbuiltin(cp, (Shbltin_f)SYSTRUE->nvalue.bfp, NIL(void*)), NV_BLTIN|BLT_DCL);
 }
 
 /*