fix server.start return nil points (#379)

Co-authored-by: luyongjie <luyongjie@37.com>

core/proc/shutdown+polyfill.go

@@ -5,11 +5,11 @@ package proc
 import "time"
 
 func AddShutdownListener(fn func()) func() {
-	return nil
+	return fn
 }
 
 func AddWrapUpListener(fn func()) func() {
-	return nil
+	return fn
 }
 
 func SetTimeoutToForceQuit(duration time.Duration) {