Security Checks to Greet Fallujah's Returning Residents   FALLUJAH, Iraq, Dec. 9 -- When the residents of Fallujah begin trickling back to their devastated city, they will be routed through sandbagged checkpoints where U.S. and Iraqi troops will take their fingerprints, issue ID cards and in some cases scan their irises, part of an elaborate plan to keep insurgents out of the former radical militant stronghold.