[Groff] default color settings

[Bernd Warken]

The handling of the default color setting in groff_out isn't clear yet.
At least, there should be a "color::set_default()" method.  Furthermore,
a copy constructor "color::color(color &)" - or a copy operator
"color &color::copy()" would be quite handy.

For the former documentation of the Df command in groff_out(5) reads:

       Df n\n Set the shade of gray to be used for filling  solid
              objects  to  n;  n must be an integer between 0 and
              1000, where 0 corresponds solid white and  1000  to
              solid  black,  and  values in between correspond to
              intermediate shades of gray.  This applies only  to
              solid  circles,  solid ellipses and solid polygons.
              By default, a level of 1000 will be used.  Whatever
              color  a  solid  object  has,  it should completely
              obscure everything beneath  it.   A  value  greater
              than  1000  or  less  than 0 can also be used: this
              means fill with the shade of gray that is currently
              being  used for lines and text.  Normally this will
              be black, but some drivers may  provide  a  way  of
              changing this.

This suggests that for values <0 or >1000 the fill color "pr->fill"
should be copied from "pr->col" that is set by the new `m' color
command.  But a simple default setting would be a valid extension as
well.

Bernd Warken