If an incoming message contains no CHRS/CHARSET control line *and* the message
text contains valid UTF-8 character encodings, set the FTN charset value to
UTF-8 so the message will be displayed/handled accordingly.
I did not add checks for header fields (to/from/subject) - we should probably
auto-detect UTF-8 in those as well, but for now, I don't see messages coming
into FidoNet echoes with UTF-8 in the header fields.
Incremented SBBSecho/EchoCfg version to 3.10.