Don't enable hysteresis if a kernel fuzz is set
As of 1523d8bb we extract the kernel fuzz and set the
LIBINPUT_FUZZ_ property instead. This stops the kernel from fuzzing and lets libinput use a different approach that should be better.
However, when the kernel fuzz is set but no
LIBINPUT_FUZZ property is set, something has gone wrong. If we end up using the fuzz as hysteresis margin, we get a double-fuzzing and cause bad accuracy and slow reaction. Better to just throw an error message in this case and let the kernel be the only one to fuzz things.
Related to #312 (closed)