Warn about a SYN_DROPPED right after finishing a sync

Warn about a SYN_DROPPED right after finishing a sync

If the first event after a completed device sync is a SYN_DROPPED, warn the
user that they're not fast enough handling this device.

The test for this is rather complicated since we can't write SYN_DROPPED
through uinput so we have to juggle the device fd and a pipe and switch
between the two at the right time (taking into account that libevdev will read
events from the fd whenever it can).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>