[Nmh-workers] Better repl -group support

[Ken Hornstein]

So CLEARLY I'm a glutton for punishment, and I'd like to bring up something
that I saw was a point of contention from the old list archives ... namely,
repl -group.

Right now repl -group out of the box pays attention to the Mail-Followup-To
header, possibly the most controversial email header of all time.  I was
initially a fan of this header, but it seems time has proven the opponents
correct; IMHO it's a) problematic to implement, and b) superceded by better
things (see RFC 2369).

Anyway, I'd been seeing the List-* headers as specified in RFC 2369.
I was thinking it would be useful to support them, specificially things
like List-Post.  Obviously this would only work for mailto: URLs.  I think
it could be done with only a few new mh-format instructions.  Thoughts?
This would let you do things like repl -list.  Or whatever; still thinking
about what the UI would look like.

--Ken