Add support for FreeBSD's libusb reimplementation

FreeBSD reimplemented the libusb API, but their
implementation of libusb_set_debug() expects an
enum libusb_debug_level instead of an
enum libusb_log_level. This causes excessive
logging on FreeBSD drowning out all expected
output.

To keep the changes to other platforms minimal
the FreeBSD specific code is kept inside an
ifdef block and the enum values are recreated
as magic numbers just like the log levels for
other platforms.