Improve UPDATE/DELETE WHERE CURRENT OF so that they can be used from plpgsql

Improve UPDATE/DELETE WHERE CURRENT OF so that they can be used from plpgsql
with a plpgsql-defined cursor.  The underlying mechanism for this is that the
main SQL engine will now take "WHERE CURRENT OF $n" where $n is a refcursor
parameter.  Not sure if we should document that fact or consider it an
implementation detail.  Per discussion with Pavel Stehule.