config check idea: check whether Apple Mail/Thunderbird are configured to either use GPGMail and/or S/MIME

[kristovatlas]

Checking for GPGMail should be fair straight forward -- already checking for some configs related to this at present. I don't have any experience with S/MIME in OS X mail clients, however, and don't want to dictate that users use one or the other to encrypt mail.

Help wanted.