better managed ownership (two new functions in level.h)

src/potion.h

@@ -2,6 +2,7 @@
 #define __POTION_H__
 
 #include <vector>
+#include <memory>
 #include "fraction.h"
 #include "output.h"
 
@@ -40,7 +41,8 @@ public:
 };
 
 typedef std::vector<potion *> potion_list;
+typedef std::vector<std::unique_ptr<potion> > potion_own_list;
 
-potion *get_potion_at(const position &pos, potion_list &plist);
+size_t get_potion_at(const position &pos, const potion_list &plist);
 
 #endif