listusb: Split CDC listing into a separate file.

listusb: parse UVC devices

- Various cleanups to the USB-audio side, which is similar
- Add in the UVC (USB Video) specific info with parsing of some of the
descriptors (most of the "control" part).

listusb: split out USB audio dumping to own file.

The code to dump audio descriptors is now twice as long as the other
parts of the listusb code. Move it to its own file for clarity.