Go back to making MimeMessage.Verify*() specific to DKIM-Signature headers
Go back to making MimeMessage.Verify*() specific to DKIM-Signature headers

It turns out that verifying/validating ARC is more complicated and it
would not make sense to validate ARC seals/signatures using the same
API.

ARC seals/signatures need to be validated as part of a set.

jstedfast/MimeKit,jstedfast/MimeKit,jstedfast/MimeKit