In the great Southern tradition of storytelling, the city of Charleston, S.C., is the principal “character’’ in Pat Conroy’s new novel, “South of Broad.’’ Like the Southern Gothic masters, William Faulkner and Flannery O’Connor, Conroy understands that a compelling sense of place will lend grace to his narrative, inhabiting the minds of his readers like the mournful strains of an old folk song. For Faulkner, it was the mythical Yoknapatawpha County in Mississippi, and for O’Connor, the mean little towns and desolate rural stretches of Georgia. Conroy’s Charleston is “the kingdom of snake handlers and clay eaters and moonshiners, where the farmland itself was stringy, stone-pocked, and unforgiving.’’