The status as a teen idol of this blond, blue-eyed heartthrob leading man of the 1960s lasted longer than the memory of many of his films. Troy Donahue is perhaps best recalled for his film role as Sandra Dee's beau in "A Summer Place" (1959) and for his starring roles as Sandy Winfield, one of the three sexy young detectives, on "Surfside Six" (ABC, 1960-62) and as Philip Barton, hotel social director, of "Hawaiian Eye" (ABC, 1962-63). For all his many screen appearances, the most lasting will undoubtedly be one of his smallest--that of Merle Johnson (also Donahue's real name), prospective