o Add sysctls to enable/disable potentially dengerous key combinations, like
        reboot/halt/debug.
o Add support for most key combinations supported by syscons(4).

Reviewed by:	dumbbell, emaste (prev revision of D747)
MFC after:	5 days
Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271381 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f