Providing yet another reminder that the Baby Boomers have settled into middle age, Howdy Doody is turning 40. And TV's original chip off the old block is about to stage a comeback. Buffalo Bob, Clarabell, Chief Thunderthud and Princess Summerfall Winterspring are set to return for a Thanksgiving two-hour syndicated TV special "Howdy Doody's 40th Birthday." Fries Distribution Co. has invited Ron Howard, Sally Field, Cybill Shepherd, John Ritter and Candice Bergen to sit in the Peanut Gallery. TV trivia buffs note: The original show made its debut on NBC on Dec. 27, 1947, and remained on the air until 1960.