* ECB&aposs Draghi fails to give specific on debt purchase plan* European shares close down 2.4 pct* Oil plunges as Saudi producers jump into supply battle(Adds European market close; changes byline, dateline previously LONDON)By Chuck MikolajczakNEW YORK, Oct 2 (Reuters) - Stocks worldwide slumped on Thursday after European Central Bank President Mario Draghi failed to provide jittery markets wit