Stop speeding up time rather than canceling Autonav in the presence of danger

I implemented one of my requests here:

http://forum.naev.org/viewtopic.php?f=6&t=916

Well, not quite. Ideally, I would like the speed resetting to happen simply by hostile ships existing nearby. But this is a good start, I think.

I did some basic testing and confirmed that it works as it should. It's still imperfect; I think in the future, I will add a time after the speed is reset (maybe 10 seconds or so) during which speed can't be increased by Autonav.