Construct a signal set and add one signal.
basic_signal_set( boost::asio::io_service & io_service, int signal_number_1);
This constructor creates a signal set and registers for one signal.
io_service object that
the signal set will use to dispatch handlers for any asynchronous
operations performed on the set.
The signal number to be added.
This constructor is equivalent to performing:
boost::asio::signal_set signals(io_service); signals.add(signal_number_1);