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

src/player.h

@@ -8,13 +8,13 @@ enum game_command : int;
 class player_base: public character {
 protected:
         int gold_cnt;
-        potion_list potions;
+        potion_own_list potions;
 public:
         player_base(RNG *rng, const feature enabled_features,
                     const enum race &nrace);
         virtual long_result move(level *lvl, const position &p);
 
-        virtual long_result apply(potion *p);
+        virtual long_result apply(std::unique_ptr<potion> p);
 
         virtual long_result attack(character *ch) override;
         virtual long_result get_hit(character *ch, const int tATK,