Add a new EV_SYN code, SYN_DROPPED, to inform the client when input
events have been dropped from the evdev input buffer due to a
buffer overrun.  The client should use this event as a hint to
reset its state or ignore all following events until the next
packet begins.
Signed-off-by: NJeff Brown <jeffbrown@android.com>
[dtor@mail.ru: Implement Henrik's suggestion and drop old events in
 case of overflow.]
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>