(mime-edit-decode-multipart-in-buffer): Use `pgp-encrypted' or `pgp-signed'

(mime-edit-decode-multipart-in-buffer): Use `pgp-encrypted' or `pgp-signed'
for the value of `subtype' if the protocol "application/pgp-encrypted" or
"application/pgp-signature" is declared; attempt to decrypt the encrypted part
with the new function `mime-edit-decrypt-application/pgp-encrypted' and strip
sender, recipients and useless tags from the decrypted part.

(TopLevel): Autoload "mime-pgp" for the function
`mime-edit-decrypt-application/pgp-encrypted'.