Since Johnny Cash’s death in September 2003, there have been four posthumous albums of his songs. One might think that, two decades on, all his major recordings had been issued. But over and again, new material keeps cropping up. Now, the country star’s son John Carter Cash has discovered unreleased demo recordings from 1993, all put together in a new album, “Songwriter.” It starts the 11 songs with “Hello Out There,” instantaneously setting the atmosphere, and you can almost believe that the “Man in Black” is greeting us from the great beyond. The sound is unmistakeably Johnny Cash. In the ea…