session keys followup, version 3

[Daniel Kahn Gillmor]

The first part of the session-keys series has already landed --
thanks to everyone who reviewed it and shepherded it on its way!

This is a respin of the remaining patches in the series, introducing
very minor changes from the previous series: typo corrections, and a
fix to the test suite to mark session-key-dependent subtests as broken
when notmuch is built against an older version of GMime.

Happy hacking,

      --dkg

history: 

v1 was introduced in id:20171025065203.24403-1-dkg at fifthhorseman.net

v2 was introduced in id:20171130085946.11332-1-dkg at fifthhorseman.net

the first 6 patches of v2 have already landed, so the numbers in this
series are 6 less than previously ("v3 01/15" corresponds to "v2
07/21", etc)