Send a QUIT instead of RSET at session end when doing 'whom'.

Send a QUIT instead of RSET at session end when doing 'whom'.

When running 'whom -check' (which really invokes post(8)), at the
end of the SMTP session we would send a RSET instead of a QUIT.  This was
technically a RFC violation (RFC 5321 says a QUIT has to be the last
thing you send), and this would cause some SMTP servers to complain.
So make sure if we're being invoked by whom to send a QUIT at the end
of the session.  Reported by Ralph Corderoy.