devq_device_get_devpath_from_fd(): Add support for FreeBSD 8

FreeBSD 8 doesn't have libprocstat. In this case, we walk the /dev/dri
directory and compare the stat(2) of each child to this directory to the
stat(2) of the given file descriptor.

Of course, this version is specific to DRM devices, our current primary
target, and needs a better solution.