Paul Natkin/Getty ImagesMojo Nixon, the singer, songwriter, and screwball radio personality known for novelty cult hits like “Elvis Is Everywhere” and “Don Henley Must Die,” died on Wednesday aboard a country music cruise. He was 66.Nixon had been co-hosting the Outlaw Country Cruise, an annual voyage run by the eponymous Sirius XM radio channel where the motor-mouthed musician had made his home for the past two decades, according to a Rolling Stone profile last year. His cause of death was an u