Overhaul sbbs_t::netmail() for FidoNet netmail:

- writes outbound netmail to SMB rather than FTS-1 Stored Messages (*.msg)
  this means you must have SBBSecho v2.30 or later in order to export FTN
  netmail (for file attachments, you'll need SBBSecho v3.04 rev 3.78 or later)
- removed support for the now deprecated "default address" feature
  (nowadays, people are much more likely to gate FidoNet over Internet rather
  than the reverse!)

The QWK-to-NetMail function still needs a similar overhaul.