On Windows, also call shutdown() while closing the client socket.

Further experimentation shows that commit 6051857fc is not sufficient
when using (some versions of?) OpenSSL.  The reason is obscure, but
calling shutdown(socket, SD_SEND) improves matters.

Per testing by Andrew Dunstan and Alexander Lakhin.
Back-patch as before.

Discussion: https://postgr.es/m/af5e0bf3-6a61-bb97-6cba-061ddf22ff6b@dunslane.net

src/backend/libpq/pqcomm.c

@@ -294,7 +294,8 @@ socket_close(int code, Datum arg)
 		 * not yet sent to the client.  (This is a flat-out violation of the
 		 * TCP RFCs, but count on Microsoft not to care about that.)  To get
 		 * the spec-compliant "graceful shutdown" behavior, we must invoke
-		 * closesocket() explicitly.
+		 * closesocket() explicitly.  When using OpenSSL, it seems that clean
+		 * shutdown also requires an explicit shutdown() call.
 		 *
 		 * This code runs late enough during process shutdown that we should
 		 * have finished all externally-visible shutdown activities, so that
@@ -302,6 +303,7 @@ socket_close(int code, Datum arg)
 		 * Windows too.  But it's a lot more fragile than the other way.
 		 */
 #ifdef WIN32
+		shutdown(MyProcPort->sock, SD_SEND);
 		closesocket(MyProcPort->sock);
 #endif