Added silly little program to dump the set of subscriptions registered
on an elvind server.  Uses the "quench" feature to grab this set.  Note
that there is no way to just ask for everything, you have to specify the
subscription attribute you are interested in.  However, since the testbed
elvin library wrapper always adds certain fields, just asking for one of
these almost certainly gets you everything you are looking for.  The only
exception (missed items) would be custom subscriptions created directly
via the elvin library that do not include one of these common fields.  I
don't think such subscriptions are ever created in practice.