[small patch] Too much ribis after tunnel entrance are set

[gerw]

When you build a tunnel with pressed ctrl, the tile after the tunnel entrance will have a double ribi (5 or 10). Therefore you can't build an slope on this tile e.g.

The patch will fix this behaviour.

[Fabio]

You couldn't build a bend, either. Or, better, the bend was possible, but it become a three way intersection. Should this behaviour be fixed by this patch as well?

[gerw]

Yes, this would also be fixed. (I always deleted the first part of an tunnel in order to build a curve, too).