Use actor_types *actor_def naming everywhere

[lukehorvat]

Hey, I was reading through the actor code and found it difficult at times to distinguish between actor entities (the instances operating within the game world) and actor defs (the declarative actor data read from xml on initialization).

Particularly in actor_scripts.c, there are many functions that have actor_types *act in the signature, and then others in the same file that have actor *act. I also found one case in actor_init.cpp where there is both actor_types *a, actor *act in the signature.

I think it'd be nice for readability to make it explicitly clear when you're dealing with an actor def instead of an actor entity. So I renamed all actor_types *act to actor_types *actor_def. Hope you agree.