*.js: Make emacs modelines consistent

js2-mode is no longer developed and we recommend js-mode these days,
so switch the modelines to specify that, and make them consistent
across all files.

https://bugzilla.gnome.org/show_bug.cgi?id=660358

js/gdm/powerMenu.js

@@ -1,5 +1,5 @@
-/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*-
- *
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+/*
  * Copyright 2011 Red Hat, Inc
  *
  * This program is free software; you can redistribute it and/or modify