Walgreens is seeking state approval to open medical clinics inside 16 of its Massachusetts stores, a sign of competition for CVS's MinuteClinics. Walgreens wants to open a Take Care clinic inside a store in Boston, potentially setting up a showdown with Boston Mayor Thomas M. Menino, who opposes the clinics. Menino has argued that retail clinics providing episodic care will fracture the medical system and ultimately hurt patients.