NIAMEY (Reuters) - Four Frenchmen held hostage in the Sahara desert by al Qaeda-linked gunmen for three years left Niger on a French government plane on Wednesday morning. The men, who were kidnapped in 2010 while working for French nuclear group Areva and a subsidiary of construction group Vinci in northern Niger, were freed on Tuesday after secret talks. A Reuters correspondent at Niamey airport said the four men boarded the jet with two French ministers, including Foreign Minister Laurent Fabius, dispatched to pick them up. "I am very happy. ...