support switch-type proximity sensors

Support binary, switch-type proximity sensors. They are are exposed over the input subsystem, emitting the SW_FRONT_PROXIMITY event code. Discussions with the kernel iio-subsystem maintainers confirmed that they really are a best fit in the input subsystem, not iio [1].

[1] https://lore.kernel.org/linux-iio/ZVevR_ajeB1jfDS9@tp440p.steeds.sam/T/#u

Fixes #363 (closed)