Some cleaning up in group handling in db.c in prep for artist groups
Don't cast to char with malloc and some error handling
Ignore duplicate tags in DAAP requests
Include groups table in database purge
In group?group-type=artists type queries return better persistent id
- this is a temporary solution until real persistent id's can be
used. This change uses a hash of album_artist instead of returning
a fixed value of 1. Thank you SquallyDoc, author of Retune, for
helping out with this and suggesting this solution.