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.