(Reuters) - Traders on Wednesday added to bets that the Federal Reserve will deliver a half-a-percentage-point reduction at its next meeting, after data showed job openings in July fell to the lowest level in three and a half years.    Traders still see a quarter-point reduction at the Fed's Sept 17-18 meeting as the slightly more likelier move, but give a 50-basis-point cut about a 49% chance, up from 41% immediately before the data, rate futures contracts show.