RIYADH, Nov 19 (Reuters) - Taking cover behind desert shrubs, Saudi commandos creep up to a house where suspected militants are holed up. With a burst of gunfire and flash bombs, they storm through doors and windows, clamber over walls and race through the rooms to take control of the gunmen's hideout. Today it's just a training drill. But men from this unit of Saudi Arabia's Special Security Forces, on the front line of the kingdom's battle with an Islamist insurgency, were in action for real two weeks ago when they raided a house in Riyadh and killed a suspected...