The Muppets have never been the most loose-limbed performers. Sitting on a person’s forearm tends to limit one’s flexibility. In the new movie “The Muppets,” however, Kermit and company bend over backward to make sure that everyone — and I mean everyone — is happy. How else to explain a movie that includes a juvenile gag about shoes made from whoopee cushions, an ironic reference to “Benson” reruns, and a parade of celebrity cameos ranging from Mickey Rooney to Ken Jeong?