Turn off connected Raspberry Pis using PEN (3B+) or RUN (3B) pins from a GPIO

[stapelberg]

See https://forum.core-electronics.com.au/t/raspberry-pi-3b-pen-run-headers-clarification/2741 and https://www.raspberrypi.org/forums/viewtopic.php?t=207961 for more details.

I calculated that the power savings amount to 2 CHF/year, so this is low priority (especially as it adds complexity), unless we need programmatic reset at some point, too.