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

For those messages, compute a synthetic Message-ID based on the SHA1
of the whole message, in the same way that notmuch would do. See:
http://git.notmuchmail.org/git/notmuch/blob/HEAD:/lib/sha1.c

To do the above, rewrite get_message_id() to accumulate header lines,
parse them to check for Message-ID, and fallback to SHA1 computation
if it is not present.

Thanks to:
- Jan N. Klug for preliminary versions of this patch
- Tomi Ollila for suggesting an elegant implementation