Get off-map vehicles from the MAPBUFFER directly.

Also forces off-map vehicles connected to on-map ones to load by
traversing the graph of vehicles every turn (might be a bit excessive,
maybe? Doesn't seem to slow things down appreciably in normal gameplay).

Also lets off-map vehicles generate power and consume fuel if they're
still on a submap that's loaded; most of the time, this will be just the
few vehicles actually connected to an active power grid, but it might
include some others. It's at least as confusing as vehicles that don't
consume fuel outside the reality bubble, even though their engines are
on.
Make fuel_left also recurse for remote battery.

Only when figuring out whether to turn on consumers, though. Defaults to
off.