Showtime's "In the 20th Century" is a millennium-related strand of feature-length documentaries in which famous directors take on major subjects of their choosing. In the fourth of the six films, "The Pursuit of Happiness," filmmaker Robert Zemeckis delves into the history of America's relationship with mind-altering substances over the past 100 years, presenting interviews with historians and professionals in the drug treatment field, interspersed with a treasure trove of film and television clips depicting the highs and lows of smoking, drinking and drugging in the 20th century