CLOSESOCKETFUNCTION: added

Introduced the initial setup to allow closesocket callbacks by making
sure sclose() is only ever called from one place in the libcurl source
and still run all test cases fine.

lib/transfer.c

@@ -2185,7 +2185,7 @@ static CURLcode Curl_do_perform(struct SessionHandle *data)
           if(CURL_SOCKET_BAD != conn->sock[SECONDARYSOCKET]) {
             /* if we failed anywhere, we must clean up the secondary socket if
                it was used */
-            sclose(conn->sock[SECONDARYSOCKET]);
+            Curl_closesocket(conn, conn->sock[SECONDARYSOCKET]);
             conn->sock[SECONDARYSOCKET] = CURL_SOCKET_BAD;
           }
         }