Use base train limits

[NathaU]

Found a way to use the base train limits when dispatching. Using the LTN limit signal is priority. If not given, check if a vanilla train limit is set, subtract the amount of vanilla trains reserved for that station and treat the result as train limit. Of course the dispatcher won't send a train to this delivery even if the vanilla train would have left the requester long time before the LTN train arrives. But I think it's a good workaround to use the base train limits in the same way the LTN signal does. It's not causing more problems than already exist when mixing vanilla and LTN trains, but some of them could be solved.