CURLOPT_SOCKOPTFUNCTION: return proper error code

When the callback returns an error, this function must make sure to return
CURLE_ABORTED_BY_CALLBACK properly and not CURLE_OK as before to allow the
callback to properly abort the operation.

lib/connect.c

@@ -926,7 +926,7 @@ singleipconnect(struct connectdata *conn,
                                CURLSOCKTYPE_IPCXN);
     if(error) {
       sclose(sockfd); /* close the socket and bail out */
-      return res;
+      return CURLE_ABORTED_BY_CALLBACK;
     }
   }