Road propagation should ignore roads when constructing roads

[Radek Liška]

http://prntscr.com/33g78u - the existing roads should be seen as "blocked area" by the pathfinding.

Now it tries to use the existing roads to "shorten" the newly-built distance, which makes road building harder :-) see http://prntscr.com/33g91z

[dalerank]

fixed #65, wrong path when road build propagation

→ <<cset 1bbdd11c717a>>