The celebrity power fueling “Years of Living Dangerously ” Showtime’s multipart  James Cameron-Arnold Schwarzenegger-Jerry Weintraub-produced documentary devoted to sounding alarms about climate change  is inevitably a double-edged sword Big-name stars obviously call attention to a project that otherwise might be lost in the shuffle  but they also make it easy for deniers to dismiss the message because of the messengers (oh those silly tree-hugging Hollywood dilettantes — though Schwarzenegger has right-wing street cred as a recent Republican governor)