Allow Altering of Nodeaccess_grants

I had this problem, see, where we were trying to use Nodeaccess module with OG Access module, as well as some custom access modules. Disaster! The issue we were trying to solve was that almost all of our nodes were supposed to be INACCESSIBLE unless the user had a specific role. When we introduced groups, however, with private, group member-only content, those OG Access grants went into place along with Nodeaccess's - so group users had access, but then Nodeaccess gave grants to all the standard users.

There was one neat thing that mitigated the problems, however: in OG Access, there was a hook to let you alter its grants. So: we were able to, under certain circumstances, use a custom module to increase the priority of OG Access's grants. That way, we could prioritize a non-public OG standard post and make it accessible ONLY to group members.

The problem was that, in the event that we wanted to use OG Access's "public group post" thing (..in our case, "public" means "public to privileged members that would normally be able to access the content") our prioritized OG Access grants made the [lower-priority] nodeaccess grants get ignored.

What we needed was to, in those situations, also prioritize certain nodeaccess grants.

Nodeaccess, however, offers no hook alter function. I've attached a patch that adds this feature. With this tiny change in place, you can write a custom module hook to modify Nodeaccess's grants. Here's my example:

function HOOK_mr_nodeaccess_grants_alter(&$grants) {
  foreach($grants as $grant) {
    if ($grant['realm'] == 'whatever' && whatever_other_stuff()) {
      $grant['priority'] = 2;
  }
}