variuous auth_mysql fixes

This patch adds mor flexibility and security to the mysql_auth backend.
You now can omit the getgroupid and addusergroup configs if you don't need
them. The default groupname is available in addusergroup and adduser now, too.
Last but not least calls to addslashes were added to avoid SQL injection.

darcs-hash:20050514192208-9977f-d970834ca68a896e725c4911639a311217dee792.gz