Set of updates to vt(4) core part.
o Declare vt(4) drivers dataset.
o Create single static structures for all early drivers.
o Add vt(4) to be by default in the kernel consoles list.
o Create one more sysinit point, to be able to initialize memory and lock
	requirement of early drivers.
o Implement early drivers select. (Only best available will be selected).
o Fix one missed "return (0)" for VTYLOCK.
o Improve locking for cases when one driver replace another.
o Make driver replacement notification less debug-look-like.

Minor spell fixes.

Sponsored by:	The FreeBSD Foundation
Garbage collect two more unused sysinit subsystems: SI_SUB_KVM_RSRC and
SI_SUB_CLISTS.

MFC after:	3 days
Switch fb and efifb drivers to use names and new vt(4) driver probe method.

Sponsored by:	The FreeBSD Foundation
Add vt(4) driver name for ofwfb driver.

Sponsored by:	The FreeBSD Foundation
[1] Make WITHOUT_FOO alway trump WITH_FOO, regardless of the system
    default. This restores more of the historical expectations that
    were broken when we started disallowing both WITH_FOO and
    WITHOUT_FOO to be defined.
[2] Document this new behavior, and improve the documentation in
    general here.

Submitted by: sjg@ [1].
Introduce CWARNFLAGS.${COMPILER_TYPE} and use that to simplify the
thicket of .if ${COMPILER_TYPE} == "clang" that controls
warnings. Also, use CFLAGS.clang in a couple places in preference to
having a similar construct that's related to the CWARNFLAGS changes.
Simplify use of the semi-bogus EARLY_BUILD variable. Also, simplify
use of semi-bogus NO_WARNS variable. Both of these, in this case,
should collapse to WITHOUT_WARNS in a future commit.
Revert r264997 and r265026. It is not required anymore.

Sponsored by:	The FreeBSD Foundation
Switch vga drivers to use names and new vt(4) driver probe method.

Sponsored by:	The FreeBSD Foundation