Separate the "is data ready" function of xfer_seek() into an

xfer_window() function, which can return a scalar rather than a
boolean.

src/core/downloader.c

@@ -228,6 +228,7 @@ static struct xfer_interface_operations downloader_xfer_operations = {
 	.close		= downloader_xfer_close,
 	.vredirect	= xfer_vopen,
 	.seek		= downloader_xfer_seek,
+	.window		= unlimited_xfer_window,
 	.deliver_iob	= xfer_deliver_as_raw,
 	.deliver_raw	= downloader_xfer_deliver_raw,
 };