all: make procd "ctrld stop" blocks until process exited

Since procd does not block when init scripts execute stop operation, it
causes ctrld command callers (the installer, users ...) thought that
ctrld process was exited, while it does not.

See: https://forum.openwrt.org/t/procd-shutdown-issues-questions/33759

cmd/cli/cli.go

@@ -449,6 +449,23 @@ func initCLI() {
 			if doTasks([]task{{s.Stop, true}}) {
 				p.router.Cleanup()
 				p.resetDNS()
+				if router.WaitProcessExited() {
+					ctx, cancel := context.WithTimeout(context.Background(), time.Second*10)
+					defer cancel()
+
+					for {
+						select {
+						case <-ctx.Done():
+							mainLog.Load().Error().Msg("timeout while waiting for service to stop")
+							return
+						default:
+						}
+						time.Sleep(time.Second)
+						if status, _ := s.Status(); status == service.StatusStopped {
+							break
+						}
+					}
+				}
 				mainLog.Load().Notice().Msg("Service stopped")
 			}
 		},