patch to create system view that lists cursors

Attached is a patch to create a new system view pg_cursors to list all
available cursors to the current session and transaction.

The patch itself is quite analogous to the patch for pg_prepared_statements
I submitted in December.

The attributes is_holdable, is_binary and is_scrollable are exposed in the
view.

There's a TODO item that only talks about WITH HOLD cursors, however the
proposed system view lists WITHOUT HOLD cursors as well.

o %Allow pooled connections to list all open WITH HOLD cursors

Because WITH HOLD cursors exist outside transactions, this allows
them to be listed so they can be closed.

I noticed that there is no regression test for binary cursors. Should there
be one? I now create one to check the view but don't actually query it.

Joachim