emacs: add compatibility functions for emacs 23

Some of the recent changes to the emacs code have used functions
introduced in emacs 24. The functions used are read-char-choice and
setq-local. This changeset adds a file notmuch-compat.el which
contains compatibility functions so that it should work on emacs
23.

Note, since these functions are taken almost unchanged from the emacs
source they are copyright the Free Software Foundation, and the header
in the file reflects that.