shared/unit-file: make sure the old hashmaps and sets are freed upon replacement

Possibly fixes #15220. (There might be another leak. I'm still investigating.)

The leak would occur when the path cache was rebuilt. So in normal circumstances
it wouldn't be too bad, since usually the path cache is not rebuilt too often. But
the case in #15220, where new unit files are created in a loop and started, the leak
occurs once for each unit file:

$ for i in {1..300}; do cp ~/.config/systemd/user/test0001.service ~/.config/systemd/user/test$(printf %04d $i).service; systemctl --user start test$(printf %04d $i).service;done

src/basic/hashmap.h

@@ -89,6 +89,14 @@ OrderedHashmap *internal_ordered_hashmap_new(const struct hash_ops *hash_ops  HA
 #define hashmap_new(ops) internal_hashmap_new(ops  HASHMAP_DEBUG_SRC_ARGS)
 #define ordered_hashmap_new(ops) internal_ordered_hashmap_new(ops  HASHMAP_DEBUG_SRC_ARGS)
 
+#define hashmap_free_and_replace(a, b)          \
+        ({                                      \
+                hashmap_free(a);                \
+                (a) = (b);                      \
+                (b) = NULL;                     \
+                0;                              \
+        })
+
 HashmapBase *internal_hashmap_free(HashmapBase *h, free_func_t default_free_key, free_func_t default_free_value);
 static inline Hashmap *hashmap_free(Hashmap *h) {
         return (void*) internal_hashmap_free(HASHMAP_BASE(h), NULL, NULL);