Don't rely on BTN_TOUCH for "finger down", the value for that is hardcoded in
the kernel and not always suitable. Some devices need a different value to
avoid reacting to accidental touches or hovering fingers.

Implement a basic Schmitt trigger, same as we have in the synaptics driver. We
also take the default values from there but these will likely see some
updates.

A special case is when we have more fingers down than slots. Since we can't
detect the pressure on fake fingers (we only get a bit for 'is down') we
assume that *all* fingers are down with sufficient pressure. It's too much of
a niche case to have this work any other way.

This patch drops the handling of ABS_DISTANCE because it's simply not needed
anymore.

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