LONDON/NEW YORK (Reuters) - The United States is taking advantage of record-low prices of one of the world's dirtiest fuels by buying record volumes, which it intends to upgrade into cleaner products before new shipping rules take effect, trading and analyst sources say. U.S. trade sources said it recently had become economical to ship fuel oil from countries such as Russia, boosting imports of the product into the United States. This comes even as prices for high-sulphur fuel oil (HSFO) on the..