Don’t mess with Allison Janney. The Oscar- and Emmy-winning actress has long been nobody’s fool, but she takes her toughness up a notch (or nine) in “Lou,” in which she plays a stone cold survivalist named Lou who has more than a few secrets stowed away. Get too close to those secrets and you’ll be sorry, as she proves when she takes out a couple of poor saps in a cabin using a can of food as her only weapon, a scene that brings new meaning to the term “opening up a can of whoop ass.” As the film opens, Lou is not a happy camper: she has a gun to her head and she’s ready to pull the trigger, b…