emacs: Give mutlipart/{signed, encrypted} their own part handler.

emacs: Give mutlipart/{signed, encrypted} their own part handler.

This is the best way to make the displayed output for
decrypted/verified messages clearer.  The special sigstatus and
encstatus buttons are now displayed under the part header button.  The
part header button is also tweaked to provide information to user
about how to proces crypto.