Some of the biggest names in showbiz have gathered to sing their favorite        Sesame Street Songs, and to celebrate 30 years of laughter, learning and fun.    There's just one hitch: the host of the show is locked in his dressing room.     Since the show must go on, Elmo takes over as host, and Big Bird, Ernie, Bert, and their friends pitch in to make sure that the evening is a hit. With Gloria Estefan, En Vogue, The Mighty Mighty Bosstones, Rosie O'Donnell, Fugees, Shawn Colvin, Steven Tyler, Jimmy Bufett, Celine Dion, Kenny Loggins, David Alan       Grier, Chris Rock, Cindy Crawford, Tyra Banks, Richard Belzer, Jon Stewart,    and Conan O'Brien. 55 mins.