Merge some code from hexrgb.el into color.el.

Merge some code from hexrgb.el into color.el.

* lisp/color.el (color-name-to-rgb): Rename from color-rgb->normalize.
Autoload.  Add optional arg FRAME, and pass it to color-values.
(color-complement): Caller changed.  Doc fix.
(color-gradient): Rewrite for better clarity and efficiency.
(color-rgb-to-hex): Rename from color-rgb->hex.
(color-rgb-to-hsv): Rename from color-rgb->hsv.  Force hue and
saturation to zero if the value is too small.
(color-rgb-to-hsl): Rename from color-rgb->hsl.
(color-srgb-to-xyz): Rename from color-srgb->xyz.  Doc fix.
(color-xyz-to-srgb): Rename from color-xyz->srgb.  Doc fix.
(color-xyz-to-lab): Rename from color-xyz->lab.  Doc fix.
(color-lab-to-xyz): Rename from color-lab->xyz.  Doc fix.
(color-lab-to-srgb): Rename from color-lab->srgb.  Doc fix.
(color-cie-de2000): Doc fix.

* lisp/facemenu.el (color-rgb-to-hsv): Deleted; use the version in
lisp/color.el instead.
(list-colors-sort-key, list-colors-print): Use
color-normalized-values.

* lisp/faces.el (color-values): Use cond for clarity.  Doc fix.

* lisp/gnus/shr-color.el (shr-color->hexadecimal): Use renamed
function names color-rgb-to-hex, color-name-to-rgb,
color-srgb-to-lab, and color-lab-to-srgb.