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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270647 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270648 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f