accelerometer: Don't wait for new data from the sensor

Instead of waiting for new data from the sensor, which might be
a long time coming, depending on the sensor device, ask the kernel
for the last state for that particular input device.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
src/libudev/Makefile.am: update libudev revision

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
Be more careful when checking for empty files

If we want to avoid reading a totally empty file, it seems better
to check after we have opened the file, not before.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>