MFC: r267978

MFC: r267978

In order to get vt(4) a bit closer to the feature set provided by sc(4),
implement options TERMINAL_{KERN,NORM}_ATTR. These are aliased to
SC_{KERNEL_CONS,NORM}_ATTR and like these latter, allow to change the
default colors of normal and kernel text respectively.
Note on the naming: Although affecting the output of vt(4), technically
kern/subr_terminal.c is primarily concerned with changing default colors
so it would be inconsistent to term these options VT_{KERN,NORM}_ATTR.
Actually, if the architecture and abstraction of terminal+teken+vt would
be perfect, dev/vt/* wouldn't be touched by this commit at all.

Reviewed by:	emaste
Sponsored by:	Bally Wulff Games & Entertainment GmbH

git-svn-id: svn://svn.freebsd.org/base/stable/10@268037 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f