CLEVELAND — He still remembers the first gunshot. For an instant, standing on the running board of the motorcade car, he entertained the vain hope that maybe it was just a firecracker or a blown tire. But he knew guns, and he knew better. Then came another shot. And another. And the president slumped down. For so many nights afterward, he relived that grisly moment in his dreams. Now, 60 years later, Paul Landis, one of the Secret Service agents just feet away from President John F. Kennedy on t