Update vt(4) for UEFI defaults and special keys

vt(4) is the default console for UEFI boot [1], and the bitmapped
kern.vt.spclkeys sysctl has been replaced with individual kern.vt.kbd_*
enable sysctls.

PR:		193710
Submitted by:	wblock [1]
Reviewed by:	wblock
MFC after:	3 days
Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D953