tidy-up: rename Curl_safefree()/tool_safefree() to curlx_safefree()

To sync names for the same macro logic between lib and src, and to move
it to the curlx namespace, to match `curlx_free()` that it's calling.

Closes #21151

lib/slist.c

@@ -132,7 +132,7 @@ void curl_slist_free_all(struct curl_slist *list)
   item = list;
   do {
     next = item->next;
-    Curl_safefree(item->data);
+    curlx_safefree(item->data);
     curlx_free(item);
     item = next;
   } while(next);