Add "USING expressions" option to plpgsql's OPEN cursor FOR EXECUTE.

This is the last EXECUTE-like plpgsql statement that was missing
the capability of inserting parameter values via USING.

Pavel Stehule, reviewed by Itagaki Takahiro

src/test/regress/sql/plpgsql.sql

@@ -2629,6 +2629,28 @@ $$ language plpgsql;
 
 select exc_using(5, 'foobar');
 
+drop function exc_using(int, text);
+
+create or replace function exc_using(int) returns void as $$
+declare 
+  c refcursor;
+  i int;
+begin
+  open c for execute 'select * from generate_series(1,$1)' using $1+1;
+  loop
+    fetch c into i;
+    exit when not found;
+    raise notice '%', i;
+  end loop;
+  close c;
+  return;  
+end;
+$$ language plpgsql;
+
+select exc_using(5);
+
+drop function exc_using(int);
+
 -- test FOR-over-cursor
 
 create or replace function forc01() returns void as $$