basic/hashmap: add hashops variant that does strdup/freeing on its own

So far, we'd use hashmap_free_free to free both keys and values along with
the hashmap. I think it's better to make this more encapsulated: in this variant
the way contents are freed can be decided when the hashmap is created, and
users of the hashmap can always use hashmap_free.

src/basic/hashmap.h

@@ -147,6 +147,8 @@ static inline int ordered_hashmap_put(OrderedHashmap *h, const void *key, void *
         return hashmap_put(PLAIN_HASHMAP(h), key, value);
 }
 
+int hashmap_put_strdup(Hashmap **h, const char *k, const char *v);
+
 int hashmap_update(Hashmap *h, const void *key, void *value);
 static inline int ordered_hashmap_update(OrderedHashmap *h, const void *key, void *value) {
         return hashmap_update(PLAIN_HASHMAP(h), key, value);