[xfer] Generalise metadata "whence" field to "flags" field

iPXE has never supported SEEK_END; the usage of "whence" offers only
the options of SEEK_SET and SEEK_CUR and so is effectively a boolean
flag.  Further flags will be required to support additional metadata
required by the Fibre Channel network model, so repurpose the "whence"
field as a generic "flags" field.

xfer_seek() has always been used with SEEK_SET, so remove the "whence"
field altogether from its argument list.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp/http.c

@@ -223,8 +223,8 @@ static int http_rx_content_length ( struct http_request *http,
 	}
 
 	/* Use seek() to notify recipient of filesize */
-	xfer_seek ( &http->xfer, http->content_length, SEEK_SET );
-	xfer_seek ( &http->xfer, 0, SEEK_SET );
+	xfer_seek ( &http->xfer, http->content_length );
+	xfer_seek ( &http->xfer, 0 );
 
 	return 0;
 }