Automatically starting Emacs in daemon mode if the Emacs server is not
running is a matter of preference better not hard coded in
notmuch-emacs-mua. Add an option to control the behaviour.
Notmuch uses long options exclusively all around. The short options in
notmuch-emacs-mua are intentionally just a compatible subset of
mutt(1). Keep it this way, if only to make documenting the fact easy!
The Notmuch style --client long option remains, of course.
