CAutoFile: Explicit Get(), IsNull() and remove unused methods

Continues #5089 `Add IsNull() to class CAutoFile and remove operator !` by @rdponticelli, which was missing a few uses of `!`. This also removes operator FILE*, in favor of an explicit Get().

Without the "magical" behaviour, this makes CAutoFile more like a standard scoped_ptr<>, except that it wraps a FILE* instead of an object, and that it adds serialize stream functionality.