Problem: need authenticated access

Solution: use PLAIN security to start with:

* mlm_client_new () address field is 'username/password' where the
  username is a mailbox name.
* malamute.c server checks passwords according to configuration,
  see malamute.cfg 'auth' section.
* mlm_server config section specifies that server uses PLAIN security
  mechanism.
* src/passwords.cfg is test password set.

Fixes #21