Allow for easier search results processing when using apachesolr search

With the current apachesolr search results with nodeaccess turned on, group id of a discussion is returned as
* $doc->nodeaccess_og_subscriber or $doc->nodeaccess_og_admin
instead of
* $doc->og_groups or $doc->og_groups_both

Thus when $extra += node_invoke_nodeapi($doc, 'search result'); is called, it shows '0 groups' always.

There is a hook 'apachesolr_process_results' that is run after, where I would like to unset the '0 groups' and process it further. However, the key of '0 groups' is a number. I would prefer if the key of '0 groups' in the 'extra' search results, was something specific such as 'og_msg' to be able to easily change or unset this.

-      return format_plural(count($current_groups['accessible']), '1 group', '@count groups');
+      $msg =  format_plural(count($current_groups['accessible']), '1 group', '@count groups');
+      return array('og_msg' => $msg);