Update the MT state properly

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Add a function to retrieve the current slot

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Rename ER_SYNC flag to LIBEVDEV_READ_SYNC

ER dated to an earlier version of this library called libevdev_read.

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