Realtime pin monitoring does not work on Pi's < 5

[Gadgetoid]

The realtime pin monitoring uses the "pinctrl" utility supplied with Pi 5 in order to get pin state information. It's currently not available on Pi 4 (does it come with Bookworm, perhaps?), so this needs some investigation.

[lurch]

See #11 - pinctrl is a "Bookworm thing" rather than a "Pi 5 thing", and I've verified that "live mode" is working correctly (over SSH) on Pi 5, Pi 4B and Pi 1B+ (although due to #12 it's not very "realtime" on a Pi 1 😆 ).