Remove never implemented syscalls
Change the compiledef GAME and CGAME to IN_GAME_VM and IN_CGAME_VM (UI will be removed soon)
Cvar: make sure the vtable is set up when Register is called.

In the inheritance chain leading to CvarProxy, the registration was done
by CvarProxy which caused problem when the Cvar system tried to validate
values using the CvarProxy pointer because the vtable was not set.