Add decryption of PGP/MIME-encrypted parts with --decrypt.

Add decryption of PGP/MIME-encrypted parts with --decrypt.

This adds support for decrypting PGP/MIME-encrypted parts to
notmuch-show and notmuch-reply.  The --decrypt option implies
--verify.  Once decryption (and possibly signature verification) is
done, a new part_encstatus formatter is emitted, the part_sigstatus
formatter is emitted, and the entire multipart/encrypted part is
replaced by the contents of the encrypted part.

At the moment only a json part_encstatus formatting function is
available, even though decryption is done for all formats.  Emacs
support to follow.