Don't flip the ship for main-thruster deceleration during coast phase

As requested on the forum:
http://pioneerspacesim.net/forum/viewtopic.php?f=3&t=198

Could use some more testing, particularly:

* Check that this doesn't mess up AIFlyTo's ship intercept mode (where you AIFlyTo() some target ship that's doing its own thing)
* Check that this doesn't mess up AIFlyTo's planetary body collision avoidance (where you AIFlyTo somewhere that requires going past a planet and the AI needs to fly around it)