Make sure we don't send any events that may have been enqueued before we
initialized ourselves. Specifically, if we're using systemd-logind the fd
remains open when we disable/enable the device, allowing events to queue up on
the fd. These events are then replayed once the device is re-opened.

This is not the case when VT-switching, in that case logind closes the fd for
us.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>