Fix FreeBSD issue with libusb_get_port_numbers()

Fix FreeBSD issue with libusb_get_port_numbers()

Currently (as of date 20150122) an ioctl problem within the
FreeBSD kernel is preventing libusb_get_port_numbers() from working.
Hence calls to libusb_get_port_numbers() will always return 0.
This makes it impossible to establish a physical path the the usb device.

This problem has existed "forever" -
meaning that libusb_get_port_numbers() has never worked.

A fix is committed to FreeBSD "current" head  -
and will later be merged (MFC'ed) to maintenance branches.
See: https://svnweb.freebsd.org/base?view=revision&revision=277417

Additionally FreeBSD requires that devices prior to calling
libusb_get_port_numbers() have been opened with libusb_open().

The patch is "forwards-compatible".
Currently it acts specificly to libusb_get_port_numbers()
currently returning 0 on FreeBSD.
In these situations it constructs an artificial path to the device.
When FreeBSD kernels appears with proper working ioctl
supporting libusb_get_port_numbers() the code will construct
proper physical paths for newer kernels - while still generating
artificial physical paths for older defective kernels.