cleartext indexing, round 3

[Daniel Kahn Gillmor]

What follows is the third round of the latest revision of cleartext
indexing patches.

This series is rebased against current master, with the improved
option-handling code, and takes into consideration the review that
Jani did on round 2 of this series.

I welcome additional review and feedback.  Many of these patches may
be familiar the third time around and will hopefully be
uncontroversial.  Please don't hesitate to review and ack the easy
ones :)

I've also pushed this series to the "cleartext-indexing" branch at
https://gitlab.com/dkg/notmuch for those who prefer a direct git pull.
it is currently git commit 6f7f6847141db2f031b29c68d966fa13c3be2da5.

Regards,

        --dkg