gEDA-user: pcb insulator layer

[DJ Delorie]

The solution is to realize that pcb is open source.

Change it to not export vias on those layers.

> When exploring different exports with gerbv ad gv I noticed the
> postscript differed.  a polygon on layer 6 named outline also was
> output on layer 5 with the original polygons in postscript, but in
> gerber output that polygon was only on 6 (as it is defined in pcb)

Intentional.  Fabs using gerber don't need to know the outline on the
copper plots, but it's really handy to have on the postscript prints.