MAR DEL PLATA, Argentina (Reuters) - Shopkeepers raced to board up storefronts and residents fled this Argentine seaside resort on Thursday as thousands of protesters prepared marches against U.S. President George W. Bush during an Americas-wide presidential summit. Bush was scheduled to arrive late Thursday for a two-day Summit of the Americas in a country where anti-Bush sentiment runs high due to the war in Iraq and U.S.-backed, free-market policies that Argentines say pushed millions of their compatriots into poverty. "People see all the iron barricades and police on every corner and they get scared," said construction worker Hernan Brito,...