Fix a select screen crash bug when a player model is unloaded (e.g., AOF BORR).
Add a temporary solution that NULLs the model handle in model_cache before deleteModel is called.
TODO: merge model cache logic (model_cache and modellist should be checked at the same time).
Other minor fixes:
Change most of the bit field type to int in openbor.h, otherwise the compiler tends to complain when they are used as array subscripts without a cast.
Clear player keys after a scene (otherwise there's no way to skip a scene before a select txt without triggering a confirm button).
