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[David Bremner]

It turns out S/MIME encryption requires non-trivial additional code in libgmime. There is no reason to wait for that to support signtures (via emacs+openssl|epg), and verification (via notmuch-cli).

Here we also test encryption, relying on emacs message-mode facilities.

(At least) two things could be improved here:

- we are using some unholy mix of openssl and gpgsm, and it would
  probably be best to get everything working with gpgsm, since
  we are already working with gpg and gpg-agent.

- It should be possible to do S/MIME decryption on the emacs level  using e.g. smime-decrype-region.

On the other hand, it seems like supporting signature verification
is a step forward.