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/hashmap.c

@@ -1793,23 +1793,28 @@ int hashmap_put_strdup(Hashmap **h, const char *k, const char *v) {
         return 0;
 }
 
-int set_put_strdup(Set *s, const char *p) {
+int set_put_strdup(Set **s, const char *p) {
         char *c;
+        int r;
 
         assert(s);
         assert(p);
 
-        if (set_contains(s, (char*) p))
+        r = set_ensure_allocated(s, &string_hash_ops_free);
+        if (r < 0)
+                return r;
+
+        if (set_contains(*s, (char*) p))
                 return 0;
 
         c = strdup(p);
         if (!c)
                 return -ENOMEM;
 
-        return set_consume(s, c);
+        return set_consume(*s, c);
 }
 
-int set_put_strdupv(Set *s, char **l) {
+int set_put_strdupv(Set **s, char **l) {
         int n = 0, r;
         char **i;