verify signed content in smime encrypted and signed message

verify signed content in smime encrypted and signed message

* lisp/gnus/gnus-art.el (gnus-mime-display-part): Parse pkcs7
parts (bug#40397).
(gnus-mime-security-verify-or-decrypt):
(gnus-insert-mime-security-button): Handle these parts.

* lisp/gnus/mm-decode.el (mm-verify-function-alist): Add pkcs7
functions.
(mm-decrypt-function-alist): Handle them.
(mm-possibly-verify-or-decrypt): Ditto.

* lisp/gnus/mm-view.el (mm-view-pkcs7-decrypt): Handle pkcs7.

Changes:
- structure the result of mm-dissect-buffer of application/pkcs7-mime
  like a multipart mail so there is no loosing of information of
  verification and decryption results which can now be displayed by
  gnus-mime-display-security

- adjust gnus-mime-display-part to handle application/pkcs7-mime like
  multipart/encrypted or multipart/signed

- add dummy entries to mm-verify-function-alist and
  mm-decrypt-function-alist so gnus-mime-display-security correctly
  displays "S/MIME" and not "unknown protocol"

- don't just check for multipart/signed in
  gnus-insert-mime-security-button but also for the pkcs7-mime mimetypes
  to print "Encrypted" or "Signed" accordingly in the security button

- adjust mm-possibly-verify-or-decrypt to check for smime-type to ask
  wether to verify or decrypt the part and not to always ask to decrypt

- adjust mm-view-pkcs7-decrypt and verify to call mm-sec-status so
  success information can be displayed by gnus-mime-display-security

- adjust gnus-mime-security-verify-or-decrypt to handle pkcs7-mime
  right with the done changes