Simplify dotneato_postprocess interface;

Simplify dotneato_postprocess interface;
remove unnecessary nodesize functions for circo and twopi;
initialize layout state in graph_init so that it will be initialized anew
for each layout;
move aspect ratio and copying of coords from ND_pos to ND_coord out of
neato spline code;
adopt convention that if the splines attribute is set to "", no splines
are generated.