Define a new `mail-user-agent' (`notmuch-user-agent') and use it by
default. Re-arrange various routines that send mail to use this
(compose, reply, forward). Insert a `User-Agent:' header by default.
This is the real commit for this functionality this time. The
previous attempt to merge this code:
commit 57926bc7b0
was botched (by Carl Worth, not David) to include only the Makefile
change. So the build was broken until this commit that actually adds
the new file.
