Convert non-UTF-8 message header fields (to/from) to UTF-8

Some messages come from non-English locales with non-UTF-8 charsets
(e.g. CP437), so their to/from fields may contain non-ASCII/non-UTF-8
characters (e.g. "Björn Felte") - so convert these fields (to/from)
to UTF-8, as needed. Uses the new(ish) utf8_encode() function, so you
must have a recent v3.17c build.
I did not transcode message subjects as I'm not clear how they are stored
and copied here. That's probably something better handled by echicken.

web/lib/forum.js

@@ -948,10 +948,10 @@ function getMessageThreads(sub, max) {
             attr : header.attr,
             auxattr : header.auxattr,
             number : header.number,
-            from : header.from,
+            from : header.is_utf8 ? header.from : utf8_encode(header.from),
             from_ext : header.from_ext,
             from_net_addr : header.from_net_addr,
-            to : header.to,
+            to : header.is_utf8 ? header.to : utf8_encode(header.to),
             when_written_time : header.when_written_time,
             upvotes : (header.attr&MSG_POLL ? 0 : (header.upvotes || 0)),
             downvotes : (header.attr&MSG_POLL ? 0 : (header.downvotes || 0)),