notmuch-mutt: support for messages that lack Message-ID headers (v3)

[Stefano Zacchiroli]

Take #3 on adding support for messages that lack Message-ID headers to
notmuch-mutt. For context see:
- http://notmuchmail.org/pipermail/notmuch/2015/thread.html#20171
- http://notmuchmail.org/pipermail/notmuch/2015/thread.html#20079

I've rewritten the patch taking into account the suggestions by Tomi
Ollila, who suggested a much cleaner approach for processing headers
and possibly fallback to synthetic Message-IDs (thanks Tomi!).

I'll be happy to hear further comments on this patch series.
If there are none, please merge it into master.

Cheers.