HUNTSVILLE, Ala. — Once known for its cotton trade and watercress farms, Huntsville, Alabama, is now the ultimate government town: About 70 federal agencies are located at the Army’s 38,000-acre Redstone Arsenal. More than half of the area’s economy is tied to Washington spending. As the government shutdown drags into a third week, people and businesses that rely on that federal largesse for their livelihood are showing the strain. Empty parking lots and darkened offices at NASA’s Marshall Space Flight Center on Redstone have translated into vacant hotel rooms because out-of-town government workers and contractors aren’t coming.