virtual console: add keyboard mode OFF

Add a new mode for the virtual console keyboard OFF in which all input
other than shift keys is ignored. Prevents vt input buffers from
overflowing when a program opens but doesn't read from a tty, like X11
using evdev for input.
Signed-off-by: NArthur Taylor <art@ified.ca>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>