main: Add Windows Service support

I've added PIPE_EVENT_SHUTDOWN because calling g_main_loop_quit() do not work when called from another thread.
Main thread was sleeping in g_poll() so I needed some way to wake it up.

By some strange reason call close(event_pipe[0]) in event_pipe_deinit() hangs.
In current implementation that code never reached so that was not a problem :-)
I've added a conditional to leave event_pipe[0] open on Win32.

src/event_pipe.h

@@ -44,6 +44,9 @@ enum pipe_event {
 	/** a hardware mixer plugin has detected a change */
 	PIPE_EVENT_MIXER,
 
+	/** shutdown requested */
+	PIPE_EVENT_SHUTDOWN,
+
 	PIPE_EVENT_MAX
 };