syscons(4): add deprecation notice

[imp]

Yea! This is one of the last huge users of Giant. vt(4) was the rewrite to get locking right.
The one issue that has been flagged privately is the one around needing this for nvidia, so we should see if that's still the case (the person mentioning it to me started with 'it used to be the case... but I don't have that hardware anymore).

[emaste]

A couple of issues have been highlighted, e.g. there are some users looking for non-UTF-8 consoles still. My hope is that the deprecation notice helps encourage users to report any currently-unknown issues.

Currently known issues can be found via the Newcons (ugh) wiki page and bugzilla vt keyword search.

• https://wiki.freebsd.org/Newcons
• https://bugs.freebsd.org/bugzilla/buglist.cgi?keywords=vt&list_id=750929&resolution=---