emacs: make modifications to message Fcc vars buffer-local

emacs: make modifications to message Fcc vars buffer-local

Previously we globally modified these variables, which tended to cause
problems for people using message-mode, but not notmuch-mua-mail, to
send mail.

User visible changes:

- Calling notmuch-fcc-header-setup is no longer optional. OTOH, it
  seems to do the right thing if notmuch-fcc-dirs is set to nil.

- The Fcc header is visible during message composition

- The name in the mode line is changed, and no longer matches exactly
  the menu label.

- Previously notmuch-mua-send-and-exit was never called.  Either we
  misunderstood define-mail-user-agent, or it had a bug.  So there was
  no difference if the user called message-send-and-exit directly. Now
  there will be.

- User bindings to C-c C-c and C-c C-s in message-mode-map are
  overridden. The user can override them in notmuch-message-mode-map,
  but then they're on their own for Fcc handling.