Tonight, the Rolling Stones, Jon Bon Jovi, Eric Clapton, Billy Joel, Bruce Springsteen, Kanye West and others are going to play 12.12.12, a benefit concert in New York for Hurricane Sandy relief. And they’re going to be joined by Nirvana, featuring a brand-new frontman in Paul McCartney, because the world is a confusing place full of mysteries and every day keeps you guessing as to what new wonders you’ll discover. Cobain, of course, is known as the gravelly voiced, emotionally troubled musical genius who brought alternative rock to the masses. McCartney is, well, he’s pretty well-known too. But the announcement that Dave Grohl and bassist Krist Novoselic would be playing together again for the first time in 20 years with a former Beatle as their frontman is a late candidate for the biggest “huh?” of 2012 …