ABDI SHEIK SOMALI government troops and opposition supporters - including some soldiers - exchanged gunfire in Mogadishu yesterday in street clashes that broke out during a protest march over delayed elections. Demonstrators said they had been attacked first by the government forces. Residents reported sporadic shooting and said rocket-propelled grenades had also been fired. The violence, which followed fighting overnight, subsided by Friday afternoon. But it fueled concern that the military could split along clan lines. It could also strengthen an al Qaeda-linked insurgency, especially as hundreds of U.S. troops have just pulled out of Somalia. Lawmakers were due to…