Rewrite convoi_t::go_to_depot and fix all known bugs in go_to_depot.  Tested.
Fix idiot typo bug in assert (sorry, James)
Fix error with persnickety MSVC compiler
Merge branch 'emergency-depot-bugfix' of git://github.com/neroden/simutrans.git into 112.x-private-car-merge

Conflicts:
	player/finance.cc