MEXICO CITY (Reuters) - With beers in hand, hundreds of former patrons Tuesday protested the closing of Mexico's oldest cantina, where Cuban President Fidel Castro, revolutionary Che Guevara and Mexican leaders all once drank. Supporters called El Nivel (The Level) a national cultural treasure. The drinking dive, which was handed the first cantina license in 1855, closed on January 2 after losing a long legal battle against the owners of the building, the National Autonomous University of Mexico. Demonstrators, many drinking from beer cans, protested outside the padlocked door of the bar, nestled in a side street near where the...