Add VNC Authentication support based on RFC6143 section 7.2.2.

Submitted by: Fabian Freyer <fabian.freyer@physik.tu-berlin.de>
Reworked by: myself
Reviewed by: grehan, rgrimes and jilles
MFC after: 1 week.
Relnotes: Yes.
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D10818