Add a force close volume method

[thejpster]

You can only close a volume if all there are no open files or directories open from it.

You might want to force-close a volume and render any open files or directories as unusable (e.g. if the disk has been removed), so we should have a method called force_close_volume that closes the volume and any files or directories on it (saving file meta-data on a best-case basis).