Fix some `hackificator` odds and ends in `wp-admin`:

* `wp-activate.php` and `wp-admin/themes.php` don't need the closing PHP tag
* Switch single quotes for HTML attribute values to double in a few places
* Convert `include_once file.php` syntax to `include_once( 'file.php' )`
* Add access modifiers to methods/members in: `_WP_List_Table_Compat`, `Walker_Nav_Menu_Edit`, `Walker_Nav_Menu_Checklist`, `WP_Screen`, `Walker_Category_Checklist`
* `edit_user()` doesn't need to import the `$wpdb` global
* `wp_list_widgets()` doesn't need to import the `$sidebars_widgets` global
* switch/endswitch syntax is not supported in Hack
* A `<ul>` in `wp-admin/users.php` is unclosed

See #27881.

Built from https://develop.svn.wordpress.org/trunk@28500


git-svn-id: http://core.svn.wordpress.org/trunk@28326 1a063a9b-81f0-0310-95a4-ce76da25c4cd