update to latest libgpiod API

[nettings]

in API versions newer than the one on Raspbian stretch, gpiod_ctxless_event_loop_multiple() is already marked deprecated. Maybe it's better to implement our own threading and resource management and use the low-level API.

Also: think long and hard whether to pull in a non-packaged dependency (upside would be that lots of dev work is happening over at https://github.com/brgl/libgpiod). They are also working on pin parameter control, but this would mean upgrading the kernel as well, to at least 4.18 IIUC.