syscons: add deprecation notice

syscons(4) is not compatible with UEFI, does not support UTF-8, and is
Giant-locked. There is no specific timeline yet for removing it, but
support for the Giant lock is expected to go away in one or two major
release cycles. Add a deprecation notice to avoid surprises, and help
ensure that any material deficiencies in vt(4) become known.

Reviewed by: manu, markj, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47396

(cherry picked from commit fc2a3ec6fe6aa00d9be0c293c38e0ef9ac7e2b64)