About a year ago, a Los Angeles man named Adam Ripp got a call from Irwin Mazur. Mazur is the first manager of Billy Joel who had a famous falling out with the star. He was calling his pal Ripp, a filmmaker, with a piece of early Billy content — one he said he’d be free and clear to use. The content was a long-lost extended interview that a 22-year-old pre-“Piano Man” Joel gave at the college radio station of Long Island’s C.W. Post in 1971 with Mazur at his side. The student who conducted the hour-long session, Ed Mann, was Mazur’s cousin. He recorded it himself, Mazur told Ripp, and so owned the masters and thus, it seemed, the rights….