FIX: resolved issues with tunnel building (largely reverting changes made in commit 6966)

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7268 8aca7d54-2c30-db11-9de9-000461428c89
FIX: check for junctions when removing powerline tunnels too

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7269 8aca7d54-2c30-db11-9de9-000461428c89