“Carnage” is being billed as “a comedy of no manners,” and that’s it in a nutshell. When one boy hits another in the face with a stick, knocking out two of his teeth, the boys’ parents meet to discuss the situation. The victim’s parents, community-building Penelope (Jodie Foster) and blue-collar Michael (John C. Reilly), invite the stick-weilder’s parents, brittle Nancy (Kate Winslet) and boorish Alan (Christoph Waltz) over for what should be an awkward but straightforward conversation. Before long, it devolves into a four-way screaming match.