emacs: maildir-fcc: prepare for use of notmuch insert

We move some code around in preparation for the use of notmuch
insert. In particular, we move the check for a valid maildir for the
fcc to when the message is sent rather than when the fcc header is
inserted. The main motivation is consistency with the insert version
(coming later) where we cannot check the validity until send.

We allow the user some chance to correct the header; the choice here
is intended to be consistent with the insert version to come.