MFC r250335: Constify libusb_get_pollfds return

  The correct return type, per our libusb(3) man page and the libusb.org
  and libusbx projects (whose interface we intend to follow for our
  libusb), is const struct libusb_pollfd **.
MFC r250342, r251495:

  Add libusb_get_port_path wrapper

  This follows the libusbx API reference at
  http://libusbx.sourceforge.net/api-1.0/group__dev.html

  Add libusb_get_port_numbers

  libusbx deprecated libusb_get_port_path and replaced it with
  libusb_get_port_numbers.  The latter omits an extra parameter which was
  unused in the FreeBSD implementation anyway.