‘Dangerous Liaisons’ ratchets up the sexual politics while dragging out the story

For an 18th-century French novel, “Les Liaisons Dangereuses” certainly got around, inspiring a play, the movies “Dangerous Liaisons” and “Valmont” in 1988 and ’89, respectively, and the twisted teen variant “Cruel Intentions.” Now comes a Starz series prequel, “Dangerous Liaisons,” which ratchets up the sex while dragging out (and out and out) the story.