"They always live with you, people you lose like that," muses Prince William, the 35-year-old Duke of Cambridge and future king of England, whose mother, Diana, was killed in a Paris car crash 20 years ago next month. "My mother lives with me every day."HBO's hour-long documentary, "Diana, Our Mother: Her Life and Legacy" (airing Monday night) offers this rare conversation with Diana's sons, William and his brother Harry, 32, who get out old their mother's old photo albums and use [...]