Mauritanians vote to elect a president on 21 June, but the poll is likely to be overshadowed by an opposition boycott. It is the second election since President Mohamed Ould Abdelaziz seized power in 2008 - and he has worked hard since then to crack down on Islamist militants who have roamed across borders in the Sahara Desert in recent years. What are the issues? A general in the army before he took power, President Abdelaziz...