OTTAWA (Reuters) – President Barack Obama said on Tuesday he still wants to reopen the North American Free Trade Agreement, despite a warning from Canada that this would be a mistake, but he said he did not want to end up curbing trade. In an interview with the Canadian Broadcasting Corp, shortly before his visit to Ottawa on Thursday, Obama also declined to characterize oil from Canada's vast oil sands region as "dirty oil" which should somehow be curtailed. Obama had alarmed Canada during the Democratic primaries last year when he advocated renegotiating NAFTA, and he reiterated this goal on...