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.h

@@ -76,6 +76,7 @@ struct hash_ops {
 void string_hash_func(const char *p, struct siphash *state);
 #define string_compare_func strcmp
 extern const struct hash_ops string_hash_ops;
+extern const struct hash_ops string_hash_ops_free;
 extern const struct hash_ops string_hash_ops_free_free;
 
 void path_hash_func(const char *p, struct siphash *state);