hal: split C and C++ headers

[virtuald]

Sez @PeterJohnson:

I think now that we're using .hpp for C++ headers it might be a good idea to split the C and C++ portions of the headers too, so the C ones are truly C only, and all the C++ stuff is in the .hpp

Along with this then anywhere in wpilib where the C enums are used, switch them to using the C++ enums instead. This will make RobotPy happier.