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).
I don't know why it was changed, but the changed version caused
full spread rather than partial spread with the triple shot, and it
also caused the quaduple shot to be arguably worse than the triple
shot.