“Dunesday”? “Doomsdune”? Whatever the nickname, cinema operators are bracing for a box office battle royale on Dec. 18. That’s when two of this year’s biggest movies — Disney’s Marvel adventure “ Avengers: Doomsday ” and Warner Bros. and Legendary’s sci-fi epic “Dune: Part III” — are scheduled to face off in theaters. On one hand, it’s an embarrassment of riches for a business that frequently complains there aren’t enough exciting films to showcase. On the other, it seems like poor planning to