Handle PKCS#7 S/MIME messages

[Daniel Kahn Gillmor]

This series applies after the "Add tests for S/MIME PKCS#7 messages"
series, which was introduced in
id:20200428185723.660184-1-dkg at fifthhorseman.net

With this series applied, notmuch handles standard PKCS#7 S/MIME
messages (using GnuPG's gpgsm as a backend, as mediated by GMime's use
of GPGME) as well as it handles PGP/MIME messages.

In addition to showing and replying to these messages, the series
covers indexing the cleartext of encrypted messages, and understanding
protected headers.

          --dkg