libnotmuch logging overhaul v4

[David Bremner]

Obsoletes

	id:1419788030-10567-1-git-send-email-david at tethera.net

The bad news is it is 3 patches longer; the good news is that I think
those patches are just testing and make sense whether we do the rest
or not.  The first 4 patches together now have the have the effect
adding a "quiet" version of open and create, slightly perversely named
"_verbose". As always, naming things is hard.

Patches 5 through 7 are just support for patch 8,
and are unchanged from the previous version.

    [Patch v4 5/9] lib/database: add field for last error string
    [Patch v4 6/9] lib: add a log function with output to a string in
    [Patch v4 7/9] lib: add private function to extract the database for

I'm not sure about the way notmuch_database_compact is handled here;
maybe it should use an extra string argument like
notmuch_database_open.

    [Patch v4 8/9] lib: replace almost all fprintfs in library with

Unmodified from the previous series.

    [Patch v4 9/9] lib: eliminate fprintf from _notmuch_message_file_open