Replace a lot of hand-rolled/pasted SMB message text creation code
in sbbs_t::postmsg() with a call to smb_addmsg(). This fixes the problem of
messages posted using the terminal server were not subject to message hash
create or dupe checking.
Also created a (currently private) helper function called msgbase_open() which
contains a lot of boilerplate code that is usually pasted around/before/after a
call to smb_open(), so I plan to use this more later.