tweak power model a bit.

The only way to get zero power is to set resistances
to infinity -- which you can't do.   So cheat a bit.
add vertical slider
add throttle slider to nav screen
Add reverse button to nav screen
Change how throttle and steering and driving works

Instead of driving the ship around like a car, now have a slider
to control the throttle, and it steers and acts more like a boat,
(or a spaceship).

Repurposed old engineering throttle opcode for this.

By request of players.
optimize find_my_ship() a bit
make reverse throttle work