[FIX] Allow user-admins to see all the groups they can assign

When not having tiki_p_admin, one could only see the groups oneself belongs to in the listing. Now all groups are being shown.

tiki-adminusers.php can only be accessed with tiki_p_admin or tiki_p_admin_users. So I see no reason not to show all the groups to the user accessing it. Especially since a non-admin user accessing the page via tiki_p_admin_users permission, can see and assign all groups via the "wrench pop-up" -> "Add or remove from a group". So not showing them in the list is actually a violation of WYSIWYCA.

Or am I missing some weird corner case here?