* Do not display empty columns on Special:UserRights if all groups are changeable...

* Do not display empty columns on Special:UserRights if all groups are changeable or all unchangeable.
* On Special:UserRights, when you can add a group you can't remove or remove one you can't add, a notice is printed to warn you.  This is to avoid confusion like that exhibited at bug 13503; it's a fairly unusual situation in MediaWiki to be unable to reverse your own actions, so a notice seems to be appropriate.  (I changed the userrights-groups-help message to point out the meaning of the extra markup; should it be renamed?  It's a common and kind of annoying problem, we should think up some consistent way of versioning messages.  Start adding numbers to the end?)