Dovecot LMTP accepted all mail regardless of whether destination was a user, broken by ae8cd4ef, fixes #852

In the earlier commit, I added a Dovecot userdb lookup. Without a userdb lookup, Dovecot would use the password db for user lookups. With a userdb lookup we can support iterating over users.

But I forgot the WHERE clause in the query, resulting in every incoming message being accepted if the user database contained any users at all. Since the mailbox path template is the same for all users, mail was delivered correctly except that mail that should have been rejected was delivered too.