Revisit S/MIME applicability rules

[christopher-henderson]

Stemming from the comment at #744 (comment) we may want to revisit the logic at IsEmailProtectionCert

An S/MIME Certificate for the purposes of this document can be identified by the existence of an Extended Key Usage (EKU) for id-kp-emailProtection (OID: 1.3.6.1.5.5.7.3.4) and the inclusion of a rfc822Name or an otherName of type id-on-SmtpUTF8Mailbox in the subjectAltName extension.

[robplee]

So, my proposal to address this issue is to change this line: https://github.com/zmap/zlint/blob/master/v3/lint/base.go#L224

To something like this:
if l.Source == CABFSMIMEBaselineRequirements && !((util.IsEmailProtectionCert(cert) && util.HasEmailSAN(cert) || util.IsSMIMEBRCertificate(cert)) {

Where HasEmailSAN (name TBC) checks for the presence of at least one san:rfc822Name or a san:otherName of type id-on-SmtpUtf8Mailbox.

Am happy to open a PR with the above if there is agreement with my suggested change.