emacs: sign/encrypt replies to signed/encrypted messages

This is a simple approach to improving security when replying to
signed or encrypted messages. If the message being replied to was
signed, add mml tag to sign the reply. If the message being replied to
was encrypted, add mml tag to sign and encrypt the reply.

This may need configuration; I for one might want to encrypt replies
to encrypted messages, but not always sign replies to signed messages.

This still includes a slight bug: if any mml tags are added, they are
included in the region containing the quoted parts. Killing the region
will kill the mml tags too.