Seven more Walgreens -- and their wonderful billboards -- are headed for S.F.The ubiquitous San Francisco corner Walgreens store is about to grow even more so: The drugstore chain, only slightly less prevalent in the city than chinese restaurants or sand, has entered into a deal to purchase seven Rite Aid stores in the city (and five more in Eastern Idaho; that was likely the "player to be named later" in this transaction). "Acquiring these San Francisco stores improves our presence in a competi