clean up references to maia_users.user_name

[mortonda@…]

Since the per domain changes, it appears that the user_name field may not be needed in its former capacity; The users.email and maia_users.primary_mail_id specify what the login name needs to be.

As such, it would be possible to either remove the user_name field, or make it an optional display name to put a real name into.

Some references need to be cleaned up though, particularly in amavisd-maia autocreation, and anywhere else it is used as an identifier as opposed to a display item.