bug#9722: list-colors-duplicates does not exclude enough colors on Windows

[Juri Linkov]

> Well, I don't really know, but in that function that's the only thing
> it is used for. revno:59445, which introduced list-colors-duplicates
> and the use of w32-default-color-map

As you can see in revno:59445, `w32-default-color-map' used to be
a variable that contained just "System" Windows colors.  I don't know
the subsequent fate of that variable.

Hardcoding "^System" in facemenu.el is not bad as long as adding the
prefix "System" is hardcoded as well in w32fns.c.

Or we could add a new system-independent variable to hold system colors
(and add GTK colors to it too).