emacs: mua: check for misplaced secure mml tags

Emacs message-send seems to ignore a secure mml tag anywhere except at
the start of the body, and it must be followed by a newline. Since
this is almost certainly not desired we check for it, and require user
confirmation before sending.

As the setup before message-send or message-send-and-exit is getting
more complicated it is convenient to unify the two correspoinding
notmuch functions.