That dangerous young man who calls himself Paul Poitier has grown up in the 27 years since he first set foot on a New York stage. All right, perhaps not 'grown up,' since we're still talking about a narcissistic con artist of adolescent fecklessness and zero self-knowledge. But there's no doubt that he has grown in stature and, in a paradoxical way, truthfulness. This is because Paul Poitier (not his real name) has been embodied with tremulous, searching sensitivity by the screen actor Corey Hawkins in the earthbound revival of John Guare's marvelous 'Six Degrees of Separation,' which opened on Tuesday night at the Ethel Barrymore Theater, with the estimable Allison Janney and John Benjamin Hickey as Paul's plump society pigeons.