CiviMail: add the ability to queue emails directly to Postfix's spool

Shane Hill from Urban Alliance for Sustainability points to qpsmtpd's library that talks directly to Postfix outbound queue: http://svn.perl.org/viewcvs/qpsmtpd/trunk/lib/Qpsmtpd/Postfix.pm?view=log

We should consider porting parts of this library to CiviMail or (if we assume Perl dependency is not a big deal, especially that we suggest AMaViS for the inbound email) hooking CiviMail to this library as an optional sending channel. Talking to Postfix through this should be much faster than through SMTP (we should keep the SMTP option for other confgurations, though).