libnotmuch logging overhaul, round 2

[David Bremner]

This obsoletes

     id:1419615761-21581-1-git-send-email-david at tethera.net

I added the promised "save a string in notmuch_database_t" logging
backend, and ended up reorganizing things a bit, since the special
casing required for _{create,open} a database seemed to naturally come
first.

One thing that I can't overemphasize here is that this series does not
(yet) include most of the changes needed to the CLI to recover these error messages. afaict that will involve adding lots (?) of fputs to error handling paths.