daemon-windows: unlink pidfile before stopping the service.

When a OVS daemon is configured to run as a Windows service,
when the service is stopped by calling service_stop(), the
windows services manager does not give enough time to do
everything in the atexit handler. So call the exit handler
directly from service_stop().

Also add a test case for Windows services which checks for
the termination of the service by looking at pidfile cleaned
by the exit handler.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com
Acked-by: Ben Pfaff <blp@nicira.com>

lib/fatal-signal.h

@@ -31,6 +31,7 @@ void fatal_signal_fork(void);
 void fatal_signal_run(void);
 void fatal_signal_wait(void);
 void fatal_ignore_sigpipe(void);
+void fatal_signal_atexit_handler(void);
 
 /* Convenience functions for unlinking files upon termination.
  *