5218 posix definition of NULL
correct unistd.h and iso/stddef_iso.h
update gate source affected

@@ -551,11 +551,11 @@
         }
 
         /* process the arguments */
 
         for (i = mac = 0;
-            (av[i] != (char *)NULL) && (av[i][0] != (char)NULL);
+            (av[i] != NULL) && (av[i][0] != '\0');
             i++) {
                 if (!fflag && av[i][0] == '-') {
                         /*
                          *  If there is not already a "--" argument specified,
                          *  and the argument starts with '-' but does not