[development] For 4.8 inclusion

Neil (and everybody who has an interest) -- I'd like to submit the na_arbitrator package, or at least the core of it, for merging into core. I don't think what I have is a good final solution, but it is a good next step in making Drupal's node_access behave more reliably. And with that, I would then like to consider integrating the forum_access code directly into the forum module, which would make a lot of people very happy, though I'm on the fence as to whether it should actually be done.