Apparently, for S/MIME, some MUAs mismatch the protocol value of
the multipart/signed and the content-type of the signature: putting
"pkcs7-signature" in one and "x-pkcs7-signature" in the other.

Change mutt_signed_handler() to independently verify the values of the
protocol and the content-type.  This still checks for correct values but
doesn't ensure they match between the two (for S/MIME).