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