Chatmail: Require encrypted mail

[s0ph0s-dog]

This is part of a series of issues tracking chatmail support in Mox.

In order to prevent chatmail servers from being used as spam sources, they must only permit outgoing emails that are either PGP encrypted, or are Autocrypt setup messages. This functionality should be tied to the big "chatmail mode" configuration switch, since it doesn't make sense to enable or disable it separately. In addition, the upstream project is considering disallowing incoming unencrypted messages to discourage sending spam to chatmail addresses. At least for now, this should be a separate option.

I've already written an implementation of the PGP detector in https://github.com/s0ph0s-dog/gochatmail/blob/main/cmd/chatmaild/chatmail_milter.go, so this one should be relatively quick.

• Add top-level configuration options for chatmail mode and incoming-encrypted-only in domains.conf
• Add per-account configuration option for chatmail mode and incoming-encrypted-only in Account type in domains.conf
• Transplant the PGP filtering code from my repo into Mox
• Transplant the tests for the PGP filtering code from my repo into Mox
• Enforce PGP encryption based on configuration options somewhere in the last ~1600 lines of smtpserver/server.go (I haven't read enough of it to be sure where yet)