Update auth.php

In Novell eDir the group search returns strings, not arrays. Added if-statement which determines if the result is an array or a string.

Without that I received using <url>?do=check the following group infos:
You are part of the groups c, c, c, user

With that change I received the following result:
You are part of the groups cn=ITSYS,ou=dsbuk,o=bug, cn=WIKI-USER,ou=dsbuk,o=bug, cn=MOBILE-USER,ou=dsbuk,o=bug, user