Joyce GoldschmidWarren Hellman, banjo messiah? With just a half-year to go before Golden Gate Park's Hardly Strictly Bluegrass festival, free-concert backer Warren Hellman is combing the wreckage of the global financial collapse, seeking to augment the fortune that keeps his massive three-day music festival stocked with first-rate talent.According to the financial newspaper The Daily Deal, Hellman & Friedman LLC is among the bidders for iShares, an exchange-traded fund business whose owner i