Detroit's roughly 25,000 unsecured vacant houses could soon all be boarded up under a plan Mayor Mike Duggan announced today at the Mackinac Policy Conference.Most of the blighted houses that would be secured in the two-year long effort are due to be torn down under an ongoing demolition program that Duggan's office says is moving at a pace of 5,000-6,000 homes per year. Duggan tells Crain's the board-up effort set to begin Aug. 1 will help neighborhoods in the meantime.…