sinksmtp: make not supplying a '-minphase' work right

Whoops, I didn't correctly handle the default case. How embarrassing.
Add support for formatted, multi-line rejections and tempfails

This adds Conn.RejectMsg() and Conn.TempfailMsg(), which are probably
misnamed by Go conventions but 'Rejectf()' and 'Tempfailf()' are too close
to Reject() and Tempfail() for me. They take a format and arguments and
you can make a multiline reply with embedded newlines. You do not supply
the code; that is determined automatically.
sinksmtp: generate more pointed rejection messages sometimes

I want to directly mention denied consent in light of what's coming
with http://fightspam.gc.ca/.