ZScript: Virtualize CheckUse()

[Major Cooke]

I recommend virtualizing functions like these because having to override PlayerThink just for a small change can quickly lead to incompatibilities in the future. I mainly bring this up because of the morph issue that was fixed not too long ago. Because PlayerThink had to be modified internally, if mods were already using it beforehand, they may suffer incompatibilities without even knowing what's going on.

As for CheckUse itself, the only thing I'm doing is changing the angle, performing CheckUse and changing it back so I don't exactly need the internals for that. I would've made a code submission featuring a wrapper function but, considering the issues of subfunctions...