You can't say the mob wasn't warned about John Shaft. "He's a bad dude," a numbers racketeer cautions them. Now, Shaft himself will deliver that message in a way New York City's wiseguys understand. Richard Roundtree reunites with the director (Gordon Parks) and screenwriter (Ernest Tidyman) of 1971's trendsetting Shaft for Shaft's Big Score!, the second of Roundtree's three movie portrayals of the street-smart, leather-jacketed private investigator. This time, the … mehrblown-to-kingdom-come murder of a client plunges Shaft into a case that bounces him like a pinball between the 133rd Precinct and competing mobs. But the players are about to be played in this "rousing and entertaining thriller" weniger