Iraqis Vote Amid Tight Security and Scattered Attacks
Iraqis begin casting ballots for 275-member national assembly and for provincial parliaments amid scattered insurgent attacks and tight security; steady streams of people line up at checkpoints to vote; suicide bomb attack in western Baghdad kills policeman and wounds several people at checkpoint and several explosions are heard across city; voters turn out in large numbers in Kurdish areas; turn out and ease with which election is carried off are regarded as major tests of Bush administration goal of implanting democracy in Middle East; prospect of widespread Sunni boycott could mar results; map; photos (L)