Add references to vt(4) and the configuration files in /usr7share/vt where
appropriate (i.e. where syscons was already mentioned and vt supports the
feature). Comments in defaults/rc.conf are updated to match the contents
of the modified man-page rc.conf(5).

Reviewed by:	pluknet, emaste
MFC after:	3 days
Fix up races with f_seqcount handling.

It was possible that the kernel would overwrite user-supplied hint.

Abuse vnode lock for this purpose.

In collaboration with: kib
MFC after:	1 week