Display the message header's "To-Extension" value (usually the user number)

*after* the to-net-addr value (if there is one), so the header display makes
more logical sense when viewing QWK netmail sent to a QWKnet node on the
QWKnet hub system.
Update the msgtotxt() prototype to follow the norm (e.g. passing through the
mode argument to smb_getmsgtxt())