Improve last change to xfaces.c

Improve last change to xfaces.c

* src/xfaces.c (font_unset_attribute): New function.
(realize_gui_face): Improve commentary and use list instead of
bitmask.
(syms_of_xfaces): Get rid of bitmask.  Replace it by a list,
there is no reason any user should have to think about bitmasks
in Emacs lisp.