THRUSPECIES/THRUACTORS bug fix

[Gez]

Here is a fix for this bug which I am submitting to review. I don't think it'd break anything, but the physics/movement code is so complicated (I didn't even know this P_PushUp() function existed before looking into the issue) that I'd rather get some feedback on it first.

[Gez]

Skulltag tester team hasn't found any problem with it and Torr has adopted the fix into ST.

Just saying that in case this hasn't been added or commented even about because of wariness.

[Major Cooke]

Okay, I'm going to give this a shot in a little bit. If this works, hopefully it'll fix the stupid bug where playing in Speed of Doom with AEoD's Left 4 Dead mode, in map09, the instant-raise floors keep getting stuck due to zombies suddenly "stacking".