Extend the source_room field all the way into the SMTP delivery loop.

Extend the source_room field all the way into the SMTP delivery loop.
Tested adding SMTP headers directly into the outgoing message at the
moment of transmission -- it worked really well.  Temporarily adding
an "X-Citadel-Room:" header as a placeholder, but in the next couple
of commits we will make it a "List-Unsubscribe:" header.