Starter batteries

Allows vehicle controls to hold a small charge (100 units of batteries), just enough to start an engine. For quick jump-starts rather than attempting a full recharge.

There is a more elegant/accurate way to do this -- add a new function to vehicles to jump-start the vehicle (like refilling a car battery but using exactly 100 units of batteries from the players inventory instead of the whole stack). However, I have no clue how to do that, so unless someone else wants to take the time to write that, this is the second-best thing. 

I feel like this should be uncontroversial, but I've been disabused from that notion in IRC. However, I did improve upon the initial design I discussed there (after doing all the work from the old design, of course), so it might be more acceptable now.