Signal Privacy state change in spaces_og_nodeapi()

When a spaces_og preset changes, and the group privacy value changes, spaces_og_nodeapi() fires off a query to update the group posts. This gets in the way of other modules trying to manage the privacy status of group posts, because in the course of this behavior it hides whether the privacy actually changed, and how it changed.

The attached patch creates a new, possibly too-specific hook to signal the privacy change, so other modules can come along after spaces_og does it's thing and enforce their own changes using the same information.

In this case, I am trying to avoid unnecessary processing of public groups that are saved without an important preset change for OG Privacy.