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