@Agzer0 Would you please give a brief description about your changes?

Implemented the following changes for #3325:

1. Used sync.Once to restrict Listen() so that it is executed only once, avoiding multiple executions of Listen() that could lead to repetitive triggering of handlers.
2. Used waitChan to ensure that Listen() remains blocking.
3. In AddSigHandlerShutdown() and AddSigHandler(), invoked signal.Notify() everytime so that even if signals are added after Listen(), they can still be registered and listened to.