sinksmtp: fix errant uses of '%#v' to be what I actually want

Short version: I got confused about which %v modifier did what and thus
which one I wanted.
smtpd.go: add Conn.AcceptMsg() to allow custom messages on an accept

This can't be applied to HELO and EHLO because those two have some
formatting requirements. I may relax this for HELO at some point
but EHLO is probably harder; I will need to read the RFCs about
exactly what is valid in an EHLO reply.
sinksmtp: add support for more 'with ...' options: 'note' and 'savedir'

This is essentially a checkpoint before I rip the current working
implementation apart to make it easier to do things.
'note' sets a note that is logged to the SMTP log. 'savedir' sets
the save directory for the message if the conversation gets that
far.
sinksmtp: add 'set-with' rule action that just sets 'with ...' values

A set-with rule does not terminate rule checking; it's purely there to
set up 'default' values for its 'with ...' options. These values can be
overridden later by another set-with or by the non-set-with rule that
matches. This can unfortunately result in some confusing and arcane
results, but that's life; I can't think of any better semantics right
now.

(I'm reluctant to add some sort of conditional/switch-based matching
to set-with or in general, although in the abstract the idea is vaguely
tempting. The devil would be in the details and it would push condition
rules further towards being a 'real language' in some sense.)