THEY HOWL, they scratch themselves, they get excited and forget they're housebroken. But even your relatives can't spoil the fun of watching The National Dog Show hosted by the Kennel Club of Philadelphia, a Thanksgiving Day fixture on NBC since 2002. This year's best-in-show went to Swizzle (right), an Australian shepherd owned by Nancy Gagnon of Stafford Springs, Conn. Dogs instead of pigskin on Turkey Day? It's not un-American; each year eight million people tune in.