varlinkctl: send READY=1 for --more calls once we received first reply

src/varlinkctl/varlinkctl.c

@@ -587,8 +587,13 @@ static int reply_callback(
                         else
                                 r = *ret = log_error_errno(SYNTHETIC_ERRNO(EBADE), "Method call failed: %s", error);
                 }
-        } else
+        } else {
+                /* Let the caller know we have received at least one reply now. This is useful for
+                 * subscription style interfaces where the first reply indicates the subscription being
+                 * successfully enabled. */
+                (void) sd_notify(/* unset_environment= */ false, "READY=1");
                 r = 0;
+        }
 
         if (!arg_quiet)
                 sd_json_variant_dump(parameters, arg_json_format_flags, stdout, NULL);