title: Cannot add signals to boost::asio::signal_set under Windows
body: <p>I'm trying to wait for signals with boost::asio under Windows, but I cannot add the signals. This is what I tried:</p>
  
  <pre><code>boost::asio::io_service ioService;
  boost::asio::signal_set quitSignals(ioService);
  quitSignals.add(CTRL_C_EVENT);
  quitSignals.add(CTRL_BREAK_EVENT);
  quitSignals.add(CTRL_CLOSE_EVENT);
  quitSignals.add(CTRL_LOGOFF_EVENT);
  quitSignals.add(CTRL_SHUTDOWN_EVENT);
  </code></pre>
  
  <p>This gives a debug assertion for the first one ("Invalid signal or error", 0), and an invalid argument exception for the others. Am I using the wrong signal numbers? The constants I use are defined in wincon.h as follows:</p>
  
  <pre><code>#define CTRL_C_EVENT        0
  #define CTRL_BREAK_EVENT    1
  #define CTRL_CLOSE_EVENT    2
  // 3 is reserved!
  // 4 is reserved!
  #define CTRL_LOGOFF_EVENT   5
  #define CTRL_SHUTDOWN_EVENT 6
  </code></pre>