Handle relative-axis rotary-encoder input (dtoverlay=rotary-encoder) …

[w531t4]

I was implementing this on a RPI, and implementing a rotary encoder is way easier with dtoverlay than with pikeyd. That being said, the dtoverlay implementation doesn't output in KEY_LEFT, KEY_RIGHT... I've augmented things a bit to interpret getting -1 and 1 and interpreting those just as it already interpreted KEY_LEFT, KEY_RIGHT.