Define the dark luminance limit as a named constant

Define the dark luminance limit as a named constant

To make the meaning of the color-dark-p cutoff luminance clear,
define it as a named constant.  (We no longer use the somewhat
obscure 0.6^2.2 definition since it doesn't really make sense
to define the limit in gamma-compressed space.)

* lisp/faces.el (color-luminance-dark-limit): New constant.
(color-dark-p): Use color-luminance-dark-limit.