Added SMTPUTF8 (RFC 6531) support:

    1) Allow 8-bit characters in addresses.
    2) Disable RFC 2047 encoding of (all) header field bodies, via
       mhbuild -headerencoding 8bit.
    3) Have post enable it in the SMTP transaction.
    4) If enabled, add the commands to the SMTP transaction
       (MAIL FROM: <address> BODY=8BITMIME SMTPUTF8).
Some things may changed, such as the 8bit encoding algorithm name.