Setup for message repair

[Daniel Kahn Gillmor]

This series of two commits offers no functional change, but it is a
useful basis for two outstanding series that i'd like to get merged:

 * "mixed up" message mangling
 * skipping legacy-display protected headers

Both of these series include code that touches lightly on notmuch's
MIME tree-crawling code, both for rendering ("notmuch show" and
"notmuch reply") and during indexing.

I'm hoping that these changes will make it somewhat less painful to
think about merging both series.

     --dkg