I rewrote the method because what it was doing was so confusing, I
couldn't figure out whether it actually worked right or not. I think
it did, and it's only 2/3 of a milllisecond anyway (not noticeable
at all), but this new way of writing it is much clearer.
The fourth is simply a duplicate of ship_collision for bullets. A
bit redundant, but I figure it's clearer of a definition. Besides,
this opens up the door to possibly making bullets a different struct
type in the future, if that turns out to be desirable.
I honestly don't understand why these cheats were here. Preventing
enemies from moving breaks some missions and doesn't do anything
useful, and preventing enemies from firing is basically no better
than the invincibility cheat (and might even be worse, if it applies
to Sid; I didn't check).