NANTUCKET - Walter Beinecke's name is spoken with a sense of awe and an undercurrent of resentment. He's the fellow, people here will tell you, who in the 1960s awoke this sleepy little island. He's the entrepreneur, who, owning much of Nantucket's downtown and practically all of the working harborfront - jammed with more fishing vessels than tourists in those days, did the math. He figured the place could benefit more from 100 people spending $100 each than 1,000 people buying a $10 T-shirt.