BAGHDAD — U.S. and Iraqi forces zeroed in on Abu Musab al-Zarqawi over two weeks, tracking his spiritual adviser to the terrorist leader's doorstep and unleashing the airstrike that included a bomb equipped in Tucson that killed them, U.S. and Iraqi officials said Thursday. The first bomb used in the airstrike was a laser-guided Paveway GBU-12, Lt. Gen. Gary North, the top U.S. air commander in the region, told Reuters. Raytheon makes the bomb's guidance systems in Tucson