Fix interpretation of signed vs unsigned values when retrieving X

Fix interpretation of signed vs unsigned values when retrieving X
Window properties, and make sure the full value is returned when not
parsed.

New subr to export type and format information about X Window
properties to lisp.

* src/xselect.c (selection_data_to_lisp_data): Treat any data as
unsigned unless its actual type is INTEGER.
CARDINALs, in particular, are unsigned.

* src/xfns.c (Fx_change_window_property): If value is a string, ignore
any provided format and force to 8.
(x_window_property_intern): If returning value as a string, the length
is actual_size times the actual format of each element, which is not
necessarily bytes.
(Fx_window_property_attributes): New subr.
(syms_of_xfns): Declare it.