tty/vt/keyboard: define LED triggers for VT keyboard lock states

In addition to defining triggers for VT LED states, let's define triggers
for VT keyboard lock states, such as "kbd-shiftlock", "kbd-altgrlock", etc.

This permits to fix #7063 from userland by using a modifier to implement
proper CapsLock behavior and have the keyboard caps lock led show that
modifier state.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Tested-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>