MOGADISHU (Reuters) - Somali militants linked to al Qaeda said on Wednesday they had sentenced a French agent to death several days after an attempt by French armed forces to rescue him failed. Al Shabaab said in a statement the decision to kill Denis Allex, held hostage in Somalia since 2009, was unanimous and followed three years of what it called "exhaustive attempts at negotiations" over his release. "With the rescue attempt, France has voluntarily signed Allex's death warrant," the militants said in an emailed statement that was also posted on the group's official Twitter handle. ...