By Mark Lamport-Stokes PINEHURST North Carolina (Reuters) - Martin Kaymer, playing flawless golf, took advantage of a rain-softened Pinehurst layout in record style to stretch his U.S. Open lead to a commanding six shots after Friday's second round. Northern Ireland's Rory McIlroy had set the previous lowest score after 36 holes with an aggregate of 131 on the way to a runaway victory in the 2011 U.S. Open at Congressional. More...