To the uninitiated, the term activist investor can sound as if a shareholder is out on Wall Street with a picket sign and a cause. Carl C. Icahn, an activist investor par excellence,is a veteran of what the practice actually entails: buying a stake in a company and pushing for changes in management or strategy. Above all, as the genial documentary portrait “Icahn: The Restless Billionaire” explains, he’s dauntingly good at making money.