gdm: Skip locked users

Don't show locked (disabled) users in the user list. Those are either
"system" users with a shell or users disabled in the control-center.

https://bugzilla.gnome.org/show_bug.cgi?id=673784

js/gdm/loginDialog.js

@@ -488,6 +488,9 @@ const UserList = new Lang.Class({
         if (user.is_system_account())
             return;
 
+        if (user.locked)
+           return;
+
         let userName = user.get_user_name();
 
         if (!userName)