Support larger iso files (more than 2GB)

No actual PSP games are this big (a UMD can only go up to 1.8 GB), but it can be convenient for combined multi-game discs.  Fixes #5169.

Not sure what ftell() returns for > INT_MAX, so better to be safe.  The main fix is using fseeko().

Also applied to cso just in case.  It's probably not super likely to see a compressed iso > 2GB but might as well support it.

-[Unknown]