FRANKFURT — A surging euro is waking up European policymakers to the drawbacks of an ambition they have long cherished: turning the common currency into a reserve currency to rival the U.S. dollar. The greenback has been the world’s reserve currency since the end of World War II, when it took over from the pound sterling. That special status has meant that it is the currency in which most global trade is conducted and in which governments worldwide hold most of their foreign exchange reserves.