By Robert MacMillan NEW YORK (Reuters) - Billionaire investor Warren Buffett is donating a total of $37 billion -- most of his personal fortune -- to a foundation started by Microsoft Chairman Bill Gates and to several family foundations, making it the largest-ever individual charitable gift in the United States. Buffett, 75, is the chief executive of investment firm Berkshire Hathaway. He is worth an estimated $44 billion, according to Forbes magazine, making him the second-richest man behind Gates, who is worth about $50 billion. The $37 billion comprises about 85 percent of Buffett's fortune. In a letter to the...