New crypto patch series

[Jameson Graef Rollins]

Hi, folks.  This is a series of patches that constitute a complete
rework of notmuch crypto support (signature verification and
decryption of PGP/MIME messages, with emacs ui support), including
some relevant preparatory patches, on top of the notmuch/master branch
at b6862c7eb9bfb00183e568b40d77ea25ade21db2.  This work is also
available on the "new-crypto" branch of my notmuch repo:

git://finestructure.net/notmuch [new-crypto]
aa41285c5f75f8ea30d4582f6b8e1aa306f1632c

It's sounding unlikely that Carl will accept this work in to master
any time soon, due to concerns he has about how the crypto processing
affects part numbering (a concern that I think is pretty minor, and
really don't affect the usability of the new functionality, fwiw).  In
the mean time, I wanted to get these patches out there, for those of
us that depend on this functionality.

jamie.