RFC6532 support ("international email"): UTF-8 in headers, message/global MIME type

[danc86]

In this old comment I claimed that message headers are supposed to always be 7 bit, but today I learnt of RFC6532 which permits UTF-8-encoded header values.

Sup should expect to see mails with UTF-8 headers if it's running at a site which supports RFC6531 for transmitting Unicode mail (for example, Postfix has supported this since 2014).

There is also a corresponding new MIME type message/global which is like message/rfc822 but the headers are allowed to have UTF-8. In practice I haven't ever run across that, but Sup should treat it the same as message/rfc822 while also accepting UTF-8 in its headers.