Provide a configuration package for exim4 or postfix

[jdreed]

Some users want the ability to run a real MTA (i.e. one that listens on port 25 and does local delivery). We should provide a configuration package for such an MTA, so that it uses outgoing-auth whenever possible (as debathena-msmtp does, and Athena 9's sendmail did), but also so that it doesn't significantly change behavior for people who migrate from debathena-msmtp-mta to exim4 or postfix.

I think it's fine to pick one of exim4 or postfix and designate that as the Debathena supported MTA.

I think it's also fine to solve this through documentation.