HAVANA, June 15 (AFP) - With a flourish and a mighty stroke of the pen, President Fidel Castro on Saturday launched his latest defensive move against capitalism, a "populist referendum" to enshrine socialism in Cuba's constitution. In a televised ceremony following an address in the town of Cacahual, outside Havana, to a crowd of 50,000 celebrating the birthday of revolutionary icon Ernesto "Che" Guevara, Castro put his name to a copy of the constitutional amendment he says was the project of mass organizations, not his government. But members of the country's illegal but tolerated dissident groups said the "referendum" --...