Loads up the admin registration center.

• Accessed by ?action=regcenter in 1.1 and ?action=admin;area=regcenter in 2.0
• Requires either the moderate_forum or the admin_forum permission.
• Loads the Login language file and the Register template.
• Calls the right function based on the subaction.
• Defaults to the 'register' subaction if you have the moderate_forum permission or the 'settings' subaction if you don't