Forces vehicle::of_turn to zero upon load rather than leaving it
undefined.

Normally this doesn't matter because vehicle::gain_moves() will
clobber it when the map next updates all the vehicles. However, if the
player is driving really fast it's possible for a vehicle to be loaded
while the map is in the middle of updating the vehicles, where the
undefined of_turn was sometimes an issue.
Fixes butcher message

The message 'There are no corpses here to butcher' no longer erroneously appears after 'You don't have a sharp item to butcher with' message is triggered.
Merge remote-tracking branch 'reverieAlice/master' into FrankenMod
Merge remote-tracking branch 'rluddy/vehicle-undefined-initiative-fix' into FrankenMod