[nmh-workers] Autocrypt

[Thomas Levine]

I have been meaning to implement Autocrypt [1] in my nmh configuration,
but I still haven't gotten around to it, so I distribute my plans in
case someone else is more ambitious.

Basic idea is call muacrypt or pyac on files in the mh folders.
Perhaps the certificates could be stored somewhere in `mhparam Path`.
Decryption could happen in inc or similar or just on mails selected
with `mhpath`. Encryption could be included in whatnow/post/send.

[1] https://autocrypt.org/
[2] https://autocrypt.org/dev-status.html