MIME-encoded.
If any RFC822* header field is a MIME-encoded UTF-8 string, then set the
(new) auxattr MSG_HFIELDS_UTF8 flag. This will be used (soon, hopefully) to
display UTF-8 encoded header fields to users. There's a gotchas here:
- MIME-encoded header fields with other non-ASCII/8-bit charsets (e.g. CP437,
  ISO-8859) are still stored "as decoded", though the MSG_HFIELDS_UTF8 flag
  may be set *later* (which would be weird), resulting in a mixture of valid
  and invalid UTF-8 header fields. One solution would be to UTF-8-transcode all
  the non-UTF-8 header fields if *any* of them are UTF-8, but we wouldn't
  know which charset to translate *from*. Assuming CP437 isn't going to be
  correct 100% of the time - so punt for now and deal with it at display
  time. e.g. if the MSG_HFIELD_UTF8 auxattr flag is set, but an hfield contains
  invalid UTF-8 data, don't display as UTF-8 (e.g. treat as CP437). We don't
  have translations for other charsets (e.g. ISO-8859) setup yet anyway.