CLOSESOCKETFUNCTION: use the callback

Fix the return type of the callback to match close() and make use of it.

include/curl/curl.h

@@ -341,7 +341,7 @@ typedef curl_socket_t
                             curlsocktype purpose,
                             struct curl_sockaddr *address);
 
-typedef void
+typedef int
 (*curl_closesocket_callback)(void *clientp, curl_socket_t item);
 
 typedef enum {

lib/connect.c

@@ -1172,7 +1172,8 @@ curl_socket_t Curl_getconnectinfo(struct SessionHandle *data,
 int Curl_closesocket(struct connectdata *conn,
                      curl_socket_t sock)
 {
-  (void)conn;
-
-  return sclose(sock);
+  if(conn && conn->fclosesocket)
+    return conn->fclosesocket(conn->closesocket_client, sock);
+  else
+    return sclose(sock);
 }