This is the main membergroup management function. It loads up all the information, sets up the sub-actions and checks permissions before calling the appropriate function.

• Called by ?action=membergroups.
• Loads the ManageMembergroups template.
• Loads the MangeMembers language file.
• Requires the manage_membergroups or the admin_forum permission.
• Calls a function based on the given subaction.
• Defaults to sub action 'index' or without manage_membergroup permissions to 'settings'.