The cowboy gave John Wayne one of his juiciest late-career roles as a leather-tough rancher who, deserted by his regular help, hires 11 greenhorn schoolboys for a cattle drive across 400 treacherous miles. ''In The Cowboys.'' Rex Reed wrote, '' all the forces that have made him a dominant personality as well as a major screen presence seem to combine. Old Dusty Britches can act,'' Co-starring the equally memorable Roscoe Lee Browne, Colleen Dewhurst and Bruce Dern, The Cowboy is exciting proof.