new way of auth module capability checking

This changes the way of how the capabilities of the used auth module are
checked as suggested as first option in
http://www.freelists.org/archives/dokuwiki/01-2006/msg00267.html

The MySQL backend WAS NOT TESTED and probably needs some work.

darcs-hash:20060203150302-7ad00-c524cafe20499f32e76ad42b4e68d7c635f6a9e9.gz