Make auth plugin behave more like ggauth's chained.class.php

I took it on me to change the code so it behaves more like it's predecessor chained.class.php https://www.dokuwiki.org/auth:ggauth

I wanted to use "authldap:authplain" and still be able to access the usermanager for authplain - that wasn't possible before when you were logged in via authldap.

I also reworked the file to comply more with dokuwikis coding_style guidelines and corrected some copy&paste errors.