Add socketcan and python-can support to panda

[ccdunder]

This provides a bidi connection between Panda and socketcan (and/or python-can).

My initial motivation was to use some opensource tools for discovery, fuzzing, and analysis of higher level CAN msg protocols. The existing socketcan driver doesn't support SPI Panda, and this is 1/4 the size of a kernel driver.

I'm opening this as a draft to solicit early feedback. If there's interest, I'll add cmdline args and threaded msg passing before moving out of draft.