Properly abstract _node_query_node_access_alter()

Right now, this function has both database logic and business logic wrapped into it. As a result, you cannot call this as an external function in the event that you _want_ to impose access rules on site administrators.

See #1359570: Restrict node views for admins fails on taxonomy pages.

Why would you want to do so? To help users configure their access control rules properly while still acting as a "super-user".

The problem is, this portion of the code prevents this usage:

  // If $account can bypass node access, or there are no node access modules,
  // or the operation is 'view' and the $acount has a global view grant (i.e.,
  // a view grant for node ID 0), we don't need to alter the query.
  if (user_access('bypass node access', $account)) {
    return;
  }
  if (!count(module_implements('node_grants'))) {
    return;
  }
  if ($op == 'view' && node_access_view_all_nodes($account)) {
    return;
  }

This check should be performed by the calling operation, not this function, so that we can eliminate code duplication caused by improper abstraction.