Input: introduce input_inject_event() function

Create input_inject_event() function which is to be used by input
handlers as opposed to input_event() which is reserved for drivers
implementing input devices. The difference is that if device is
"grabbed" by some process input_inject_event() will ignore events
unless sent from the handle that is currently owns the device.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>