by Claudine Zap Step aside, snooty pooches with bloodlines that are older than the American colonies. No more looking down your snouts at the mixed-breed dog. The American Kennel Club has put a paw out to the American mutt, the dog of dogs, which has never been allowed to compete alongside the purebreds. These pooches are a hodge-podge of spare parts from the proudest of breeds. They may not be the prettiest, but they are trainable, smart, and obedient,