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/shared/unit-file.c

@@ -463,7 +463,7 @@ int unit_file_find_fragment(
 
         /* The unit always has its own name if it's not a template. */
         if (IN_SET(name_type, UNIT_NAME_PLAIN, UNIT_NAME_INSTANCE)) {
-                r = set_put_strdup(names, unit_name);
+                r = set_put_strdup(&names, unit_name);
                 if (r < 0)
                         return r;
         }
@@ -493,7 +493,7 @@ int unit_file_find_fragment(
                                 if (!streq(unit_name, *t))
                                         log_debug("%s: %s has alias %s", __func__, unit_name, *t);
 
-                                r = set_put_strdup(names, *t);
+                                r = set_put_strdup(&names, *t);
                         }
                         if (r < 0)
                                 return r;