minor passenger fixes

James,
I've seen problems with passengers bound to depots never being subtracted from total_freight, which would eventually "fill up" convoys without any actual freight, and dschr reports seeing something that I also think is due to this issue, which I'd previously believed to be specific to my local changes. Once again the issue is that gr->find<gebaeude_t>() doesn't find depots.

Would you be willing to consider a grund_t::get_building() function which combines gr->find<gebaeude_t>() and gr->get_depot()? I'm not quite sure how it would handle factories, though.

I hope your move is going well,
Philip