Update SPIDEV driver for Character Device v2 ABI; cache FDs

[2bndy5]

• uses Linux Kernel's Character Device v2 ABI instead of the deprecated v1 ABI
• caches the File Descriptors used to control GPIO pins

[2bndy5]

I still want to test the caching of FDs. Specifically,

• does not break other apps that use the same /dev/gpiochipX when RF24 app is running
• still works when dynamically init-ing pins with RF24::begin() instead of c'tor

I'm opening this PR as the solution for #943 will be separate from these changes implemented for non-IRQ GPIO usage.

[TMRh20]

Haven't tested everything, but looks like you've been thorough.

Works like a charm!

The new install script with automated selection for branches is prettynice to have now too for testing stuff.

[2bndy5]

The new install script with automated selection for branches is prettynice to have now too for testing stuff.

I think it would be helpful to have it git pull --rebase after git checkout <branch/tag>. That way you don't have to contend with installing from branches that are already checked out locally but have force-pushed updates on the remote.

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Anyway. Yeah, I'm ready to merge this and move on to #943. I've been testing it pretty thoroughly (even in my local pyRF24 cloned repo).

[TMRh20]

Cool, been running this on all my RPis overnight as well.