[Groff] fixed color support

[Werner LEMBERG]

I've fixed the color support.  It should now work as expected.  As
announced, color nodes are gone; they have been replaced with color
variables in the various nodes.

Additionally, the behaviour of \X (and \Y) is more predictable:
Before emitting the `x X ...' sequence,  both colors and the current
font are updated if necessary.

Please test!


    Werner