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