vt(4): Add cngrab() and cnungrab() callbacks

They are used when a panic occurs or when entering a DDB session for
instance.

cngrab() forces a vt-switch to the console window, no matter if the
original window is another terminal or an X session. However, cnungrab()
doesn't vt-switch back to the original window currently.

MFC after:	1 week
drm: Don't "taskqueue" vt-switch if under DDB/panic situation

If DDB is active, we can't use a taskqueue thread to switch away from
the X window, because this thread can't run.

Reviewed by:	ray@
Approved by:	ray@
MFC after:	1 week