Fix case when vt(4) started w/o driver assigned.

Fix case when vt(4) started w/o driver assigned.
o Always init locks and cv ASAP.
o Initialize driver-independent parts even if driver probing fail.
o Allow to call vt_upgrade anytime, for later loaded drivers.
o New window flag VWF_READY, to track if window already initialized.
Other updates:
o Pass vd as a cookie for kbd_allocate.
o Do not blank window on driver replacement.

Tested by:	hselasky (RPi), emaste(VGA, EFIFB, KMS), me

MFC after:	7 days
Sponsored by:	The FreeBSD Foundation