RPi 5 issues

as of right now, rpigpior does not work with the Raspberry Pi 5 and possibly with the bookworm version of Raspberry Pi OS. The reasons are:

- `is.rpi()` tests for a string that is no longer there. It always returns `FALSE`.
- rpigpior assumes the GPIO is on GPIOCHIP0. As of the Raspberry Pi 5, it is now on GPIOCHIP4. 

I am working on these issues.