(e.g. to, from, subject) ... when transmitting to other hosts (e.g. POP3
clients or other SMTP hosts).

For messages that contain CP437 or UTF-8 chars in these fields and were *not*
imported into the BBS via SMTP or POP3, these header fields would be
transmitted to other hosts with the raw CP437 or UTF-8 chars, thus violating
POP3 and SMTP protocols and likely resulting in garbage displayed in message
readers.

The fix is to encode (using MIME "encoded words", per RFC 2407) where
necessary.

Since moderm mail clients (e.g. Thunderbird) don't see to support CP437
charset in MIME encoded header fields, always translate to UTF-8 first.

We probably should be translating message body text to UTF-8 as well, for
maximum compatibilty with modern mail readers, but this commit doesn't address
body text issues with CP437-encoded content. That'll come later.