site admin plugin menus added using wpmu-admin.php

[wpmuguru]

All existing plugins that add menu items to the site admin menu add them to the wpmu-admin.php parent menu item. By renaming wpmu-admin.php to ms-admin.php, the menu items in existing site admin plugins are ignored.

We'll have to add some backward compatibility code to catch those and add them to the ms-admin.php menu.

[ryan]

This can be mapped in the $_wp_real_parent_file array in menu.php.

[wpmuguru]

(In [12786]) add real parent file for wpmu-admin.php, fixes #11961