UPDATE: Dave Mustaine has clarified his position, saying that he likes Santorum but does not officially "endorse" him.   I was shocked to learn that Dave Mustaine, the singer of Megadeth, endorsed Rick Santorum for president. If you're not a fan of heavy metal, you might not know that Megadeth is one of the most popular metal bands of all time -- and they're certainly one of the most political bands in any music genre.  Dave Mustaine actually covered the presidential race for MTV News in 1996. As…