Users must not be allowed to step outside the volume with subPath.
Therefore the final subPath directory must be "locked" somehow
and checked if it's inside volume.
On Windows, we lock the directories. On Linux, we bind-mount the final
subPath into /var/lib/kubelet/pods/<uid>/volume-subpaths/<container name>/<subPathName>,
it can't be changed to symlink user once it's bind-mounted.
On FreeBSD, it is perfectly reasonable to make use of util_linux.go.
Rename util_linux.go to util_unix.go so that it may be used on non-Linux
UNIX-like systems. Add proper 'freebsd' build tags.
