LONDON/ALGIERS (Reuters) - The In Amenas gas plant felt impregnable to many who worked there - walled in, hundreds of miles from anywhere and with the Algerian army constantly patrolling its desert approaches. That was a mirage. Libya, an ex-police state turned arms bazaar and now open for jihad, lies just 50 empty miles away. And in any case, the enemy was probably already inside the gates. ...