Johnny Cash rode a boyhood dream and three chords to country music stardom. But as Robert Hilburn’s definitive biography vividly chronicles, that dream spawned a sizable brood of nightmares. Cash grew up poor, mostly in Dyess, Ark., where his parents took part in a government-backed farmland “colonization” program. While poets, philosophers and singers rhapsodize about living close to the land, Cash knew better.