SEASON PREMIERE: Part 1 of 3. Frontline, the most venerable current documentary series on television, takes on a story that’s decades old, and devotes three episodes to it, beginning with tonight’s opener. The focus is the December 1988 bombing of Pan Am Flight 103, whose Germany-to-Detroit route was cut short by terrorists, with the voyage, and many lives on the plane and on the ground, ending as the plane crashed in Lockerbie, Scotland. This film is by filmmaker Ken Dornstein, whos