Check if the message is encrypted before DKIM check

[link2xt]

If incoming message is not encrypted, we can reject it by just looking at it, without doing any DNS requests.

If this check is moved above, we will not check DKIM on unencrypted message and will also not have to debug DKIM verification on messages that are going to be rejected anyway:

filtermail/src/inbound.rs

Line 88 in e1edecc

let mail_encrypted = check_encrypted(&message, false);