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.c

@@ -1768,6 +1768,32 @@ int set_consume(Set *s, void *value) {
         return r;
 }
 
+int hashmap_put_strdup(Hashmap **h, const char *k, const char *v) {
+        int r;
+
+        r = hashmap_ensure_allocated(h, &string_hash_ops_free_free);
+        if (r < 0)
+                return r;
+
+        _cleanup_free_ char *kdup = NULL, *vdup = NULL;
+        kdup = strdup(k);
+        vdup = strdup(v);
+        if (!kdup || !vdup)
+                return -ENOMEM;
+
+        r = hashmap_put(*h, kdup, vdup);
+        if (r < 0) {
+                if (r == -EEXIST && streq(v, hashmap_get(*h, kdup)))
+                        return 0;
+                return r;
+        }
+
+        assert(r > 0); /* 0 would mean vdup is already in the hashmap, which cannot be */
+        kdup = vdup = NULL;
+
+        return 0;
+}
+
 int set_put_strdup(Set *s, const char *p) {
         char *c;