Iraq's Top Cleric Arrives in Najaf NAJAF, Iraq - Iraq's top Shiite cleric made a dramatic return to Najaf at the head of a massive convoy Thursday hoping to end three weeks of fighting in the holy city, but hours earlier a mortar barrage slammed into a mosque filled with Iraqis preparing to join his march, killing 27 people.    Along with the mortar attack, another group of thousands of marchers heading into Najaf from its sister-city Kufa came under fire from an Iraqi National Guard base...