protected roads/noise barriers

James,
(for future reference: this patch allows noise barriers as discussed on the forum at http://forum.simutrans.com/index.php?topic=13872)

Here's my proposed patch.

I think we haven't discussed the change to the public right of way checking code, which I've given its own fahrer_t which cannot enter tiles with noise barriers or private entry barriers. In future, this change might also be useful for enforcing (lack of) way constraints on public routes, so you can no longer replace a river by a tub boat canal when weight limits aren't being strictly checked.

By the way, do you prefer single commits in pull requests? Rebasing them is probably easier to do from my end, but the commit history might also contain clues that might help with future bugs.

Philip