su.c: implement WinSock2 su_close() properly
su.c: implement WinSock2 su_close() properly

Use DisconnectEx() before closing.

xhook/sofia-sip,BelledonneCommunications/sofia-sip,BelledonneCommunications/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip,unispeech/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,xhook/sofia-sip