Roland Emmerich’s “White House Down” gets its New York premiere tonight. But many people saw it last week and know at least one thing: the Kennedy family will not be in the audience at any screenings. A big part of the “White House Down” story has to do with fabled tunnels that supposedly run beneath the White House. It’s where Channing Tatum takes Jamie Foxx, who plays the president, when the White House is attacked. How do they know about the tunnels? “It’s where John Kennedy took Marilyn Monroe to have sex,” says someone in the movie– I’m paraphrasing, but that’s the gist of it. Later, Tatum asks someone else if this is true, and he is assured that it is. Well, of course, it’s not. But at that point “White House Down” has unexpectedly become a comedy anyway, so we’re supposed to laugh it off. The whole tunnel thing, like JFK and Marilyn, is a myth. The latter is offered as anecdote through the ages. On the season finale of “Veep,” the best comedy on TV right now, Julia Louis-Dreyfus’s Selita makes reference to it also. It’s a brief but pungent moment.