emacs: Change FCC to be relative to notmuch mail store, not message-directory

emacs: Change FCC to be relative to notmuch mail store, not message-directory

Otherwise, FCC is too hard to use, (user must set it and also set message-
directory variable to match notmuch mail datbase path). As a rule, I'd like
for users of notmuch to not be required to muck around with non-notmuch
mail settings in emacs.

The above is only really possible now thanks to the recent addition of the
"notmuch config get" command which allows emacs to query the currently
configured notmuch database path.

This also now allows an absolute-path FCC to be set if desired.