- Use libusb20_strerror() function instead of custom usb_error() one.
- Rename "aux.[ch]" to "util.[ch]" which is a more common name for
utility functions and allows checkout on some non-FreeBSD systems
where the "aux.*" namespace is reserved.
- Fix some compile warnings while at it.

PR:		usb/183728
MFC after:	2 weeks