AROS 64bit fixes filehandles #2

Solve crashes when opening, closing and managing files for AROS 64-bit when filehandles are located in memory > 32-bit by making sure the used file handles are actually of type BPTR/THandle instead of longint/cardinal (and stays compatible to AROS 32-bit).

Sorry @chainq : I simply forgot to include this particular change in my previous filehandle fixes branch so it (wrongfully) wasn't included in the previous (merged) filehandles fix.