MFC cleanup of libusb20 example

r257779 by hselasky:

- 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.

r257796 by glebius:

Finish r257779.

PR: 183728

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add some examples about how to use FreeBSD's libusb20 in your own
code.