notification information functions

I am working on moving the notification buffer into shared memory as
previously discussed. Since pg_listener will no longer exist, I think we
need to provide a couple of information functions.

I suggest:

pg_listened_events(out event name) returns setof record
pg_pending_events(out event name, out message text) returns setof record

The first would show events being listened on by the current backend,
while the second would show all pending events for the current db.

Given that there will no longer be any central place where events will
be registered to be listened on, it will not be possible to show all
such events for the current db.

Comments?

cheers

andrew