Handle PKCS#7 S/MIME messages v2

[Daniel Kahn Gillmor]

This revision of the PKCS#7 S/MIME handling series is based on (and
very similar to) the series sent on the thread starting at
id:20200430201328.725651-1-dkg at fifthhorseman.net

However, it is rebased after more gracefully handling the subtle
errors in X.509 certificate validity when built against older versions
of GMime.

In particular, the patch found at
id:20200512222010.371054-1-dkg at fifthhorseman.net must be applied
before this series can be applied.

Feedback and critiques welcome,

         --dkg