Hook when checking for enter permissions on vehicles

[Renegade]

For #bug:733, if the game checks whether a unit can enter a vehicle to take it over separately from checking whether it should get an entry cursor, we need a hook to override the permission check.

Since the game should already perform the exact check needed when checking for VehicleThief-ness, a hook in that function would be best, to override the check from "Should this unit be allowed to enter this vehicle because it's a VehicleThief?" to "Should this unit be allowed to enter this vehicle because it's a VehicleThief or because the vehicle is driver-less?".

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Alternatively: Is it feasible to just set a unit's VehicleThief to true in the hook from bug:1123, and set it back in a separate hook if the user doesn't click?

That way, we could just use the existing VehicleThief logic and wouldn't have to deal with custom stealing code.