Permissions for managing subscriptions

Even if a user has permissions configured to allow him or her to manage his or her own subscriptions, selecting a checkbox in the 'Subscriptions' section of the 'Notifications' tab (under My account) and then trying to perform the delete action results in an error message about not having the required permissions. Here is a patch to fix this.

I changed the === to == because db_result will not necessarily return something of type integer. The only time this should be a problem is when there is an error, so that FALSE is returned by db_query. Then == will only match when $sids is empty, which should not be a problem.