mhpgp: use trusted header field copies to replace original header fields

Enigmail provides so called ``protected-headers'', which are copies
of some main mail header fields to protect them from forgery.

Mhpgp(1) works in a rough way and simply adds this MIME part
to the main header, and this resulted in a doubling of those header
fields.

Now those original headers are removed if trusted copies are
provided within the encrpyted MIME part.