Fix a regression from #7393: `last_targeted_subobject` must be updated when the player's target and target subsystem are changed, or the subsystem will be retargeted due to `hud_target_change_check`.
Also fix related issues:
- the `ship.Target` and `ship.TargetSubsystem` scripting setters now use the standard `set_target_objnum` function and also correctly handle the new target subsystem
- fix an indexing error in `order.TargetSubsystem`
Co-Authored-By: Claude Fable 5 <[email protected]>