subject header fields are non-ASCII yet valid UTF-8, set the auxattr flag
indicating that the msg header is in UTF-8.
This function is used by JS MsgBase.save_msg(), so any scripts/services that
use it (e.g. nntpservice, newslink, imapservice, etc.) to save messages with
header fields that may be UTF-8 encoded will automatically benefit from this
detection. Mixing CP437 (the default) with UTF-8 encoded header fields is not
supported, so if one header field is valid UTF-8 (not plain US-ASCII), then
they had better all be either plain US-ASCII or UTF-8 or there will be
UTF-8 decoding issues when attempting to display or convert later.