vt(4): Implement basic support for KDSETMODE ioctl

With the current implementation, this allows an X11 server to tell
the console it switches a particular window in "graphics mode". This
information is used by the mouse handling code to ignore sysmouse events
in the window taken by the X server: only him should receive those
events.

Reported by:	flo@, glebius@, kan@
Tested by:	flo@
Reviewed by:	kan@
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270702 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f