Mixed encoding messages are problematic for QWK and FTN

It's possible to post messages from Synchronet that contain CP437-encoded header fields (e.g. Subject field) and UTF-8 encoded message body. This is a problem for both QWK (Synchronet extensions for UTF-8) and FTN, which don't support this mix of encodings.

We can translate/correct the encodings upon export to these formats, but it's probably better to try to prevent them from being this way in the message base in the first place.