Allow path-bound builtins on restricted shells

If a system administrator prefixes /opt/ast/bin to the path and
then invokes the shell in restricted mode, they clearly intend for
the user to run those AST utilities.

Similarly, if a system administrator sets a PATH for a restricted
shell that includes libraries listed in the .paths file, they must
have intended for the user to use those loadable built-ins, as they
will be associated with the pathnames of their respective
libraries. Since the user cannot change PATH or use the builtin
command, they still cannot load just any built-in they choose.

src/cmd/ksh93/sh/path.c:
- Remove SH_RESTRICTED check when handling path-bound builtins
  or dynamic libaries containining builtins in $PATH.

src/cmd/ksh93/tests/builtins.sh:
- Add test verifying a restricted user can use /opt/ast/bin/cat
  via a PATH search.

Progresses: https://github.com/ksh93/ksh/issues/138

src/cmd/ksh93/sh/path.c

@@ -781,15 +781,17 @@ Pathcomp_t *path_absolute(Shell_t *shp,register const char *name, Pathcomp_t *pp
 			return(0);
 		}
 		isfun = (oldpp->flags&PATH_FPATH);
-		if(!isfun && !sh_isoption(SH_RESTRICTED))
+		if(!isfun)
 		{
 #if SHOPT_DYNAMIC
 			Shbltin_f addr;
 			int n;
 #endif
+			/* Handle default path-bound builtins */
 			if(*stakptr(PATH_OFFSET)=='/' && nv_search(stakptr(PATH_OFFSET),shp->bltin_tree,0))
 				return(oldpp);
 #if SHOPT_DYNAMIC
+			/* Load builtins from dynamic libraries */
 			n = staktell();
 			stakputs("b_");
 			stakputs(name);