For composing new messages and forwarding, leave the cursor on the
'To:' field. For replies, leave the cursor at the start of the
body. In all cases, mark the buffer as not modified so that the user
is not prompted if she decides to immediately kill the buffer.
"notmuch-address.el" tries to be careful to insinuate itself into
message mode only if it will do something useful, so it's safe to load
it all of the time.
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.
