basic/set: let set_put_strdup() create the set with string hash ops

If we're using a set with _put_strdup(), most of the time we want to use
string hash ops on the set, and free the strings when done. This defines
the appropriate a new string_hash_ops_free structure to automatically free
the keys when removing the set, and makes set_put_strdup() and set_put_strdupv()
instantiate the set with those hash ops.

hashmap_put_strdup() was already doing something similar.

(It is OK to instantiate the set earlier, possibly with a different hash ops
structure. set_put_strdup() will then use the existing set. It is also OK
to call set_free_free() instead of set_free() on a set with
string_hash_ops_free, the effect is the same, we're just overriding the
override of the cleanup function.)

No functional change intended.

src/basic/hash-funcs.c

@@ -10,6 +10,8 @@ void string_hash_func(const char *p, struct siphash *state) {
 }
 
 DEFINE_HASH_OPS(string_hash_ops, char, string_hash_func, string_compare_func);
+DEFINE_HASH_OPS_WITH_KEY_DESTRUCTOR(string_hash_ops_free,
+                                    char, string_hash_func, string_compare_func, free);
 DEFINE_HASH_OPS_FULL(string_hash_ops_free_free,
                      char, string_hash_func, string_compare_func, free,
                      char, free);