user choose which address to use as the source address when composing a netmail
message (the default being the most appropriate for the dest zone/net). This
change only works with SBBSecho v3.05 or later.
Some other incremental and safety improvements to sbbs_t::netmail() too.
FTN netmail file attachments needs some more work however (the "FA:" subject
prefix trick) - just remove it?