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.
