The only string format for colors that we now support are #rrggbb and #rrggbbaa.
All other strings will cause errors.
Thanks to this, color_init_cookie_t can be removed. There won't be a request to
the X server for transforming named colors any more and so there won't be a
cookie. This means that color_init_reply() has to be removed, too.
Signed-off-by: Uli Schlachter <psychon@znc.in>