Allow routers to control when users are loaded

The binlogrouter requires that users are not loaded at startup. This
allows it to inject the service user into the list of valid MySQL users so
that the binlogrouter can be controlled via the listeners.

server/core/dbusers.c

@@ -277,9 +277,18 @@ replace_mysql_users(SERV_LISTENER *listener)
 
     if (i <= 0)
     {
-        users_free(newusers);
-        /* Failed to load users, restore old users and resources */
-        listener->resources = oldresources;
+        /** Failed to load users */
+        if (listener->users)
+        {
+            /* Restore old users and resources */
+            users_free(newusers);
+            listener->resources = oldresources;
+        }
+        else
+        {
+            /* No users allocated, use the empty new one */
+            listener->users = newusers;
+        }
         spinlock_release(&listener->lock);
         return i;
     }