Add revert support for mulitpart signed PGP MIME

* src/mimemaker.c (restore_msg_from_moss): Don't set gpgol
  msg_class.
* src/revert.cpp (gpgol_mailitem_revert): Add support for
  MULTIPART_SIGNED.

Multipart signed is directly restore from the mosstmpl. This
is very fragile though and only works good for mails sent by
gpgol because Outlook mangles the innter MIME boundaries for
mails from at least KMail and Enigmail. So the signature is
broken after the mail went through outlook.