Signal Handler should not use non async-signal-safe functions

[azeey]

Locking mutexes and printing to the console using gzdbg are not considered async-signal-safe and could cause the application to hang or go into an undefined state.

gz-common/src/SignalHandler.cc

Lines 139 to 145 in e282a8d

	void SignalHandlerPrivate::OnSignal(int _sig)
	{
	std::lock_guard<std::mutex> lock(this->cbMutex);
	gzdbg << "Received signal[" << _sig << "].\n";
	for (std::function<void(int)> cb : this->callbacks)
	cb(_sig);
	}

Potential implementation reference: https://github.com/sphenical/signal-handler/