Add signature verification of PGP/MIME-signed parts with --verify.

This is primarily for notmuch-show, although the functionality is
added to show-message.  Once signatures are processed a new
part_sigstatus formatter is emitted, and the entire multipart/signed
part is replaced with the contents of the signed part.

At the moment only a json part_sigstatus formatting function is
available.  Emacs support to follow.

The original work for this patch was done by

  Daniel Kahn Gillmor <dkg@fifthhorseman.net>

whose help with this functionality I greatly appreciate.