Universal auth_db.php to handle authenticating users from converted boards

There is a problem with converted board in that the auth_xxx.php does not get updated when phpBB is updated. This can cause login problems.My proposed solution is to have auth_db.php check for ALL possible passwords from ALL converted boards. Attached is a diff file showing the necessary changes.Converted boards also sometimes add fields to the users table, so these fields need to be checked if they exist and build the proper SELECT query to get the user's information.