Shutdown() hangs forever when nats-server is listening to signals

[Nintron27]

If Shutdown() is called and nats-server was passed NoSigs false, but the call to Shutdown wasn't induced by a system signal, then Shutdown will hang forever because if it calls nats-server Shutdown() it will cause an internal function to panic due to nats-io/nats-server#6358.

A potential fix in the meantime would be to duplicate the signals nats-server listens for internally and if that context is cancelled then don't call Shutdown().