Implement the filelist savedata mode properly + other save fixes

With these changes, FF4 is willing to save at least, but it's still not working properly.

When you try to save or load for the first time, you get an error code pop up (this code happens to be the code we correctly return for directory doesn't exist, but even if we return 0, it shows the same code.. although it shows what we return if we return a different error code.)

However, saving/loading still works.  The message doesn't come up again once the directory does exist.

Anyway, this largely implements the FILES mode properly based on a series of tests on actual firmware.  I haven't tested everything, but it showed clearly the difference between SECURE and non-SECURE READDATA/WRITEDATA/MAKEDATA modes.

-[Unknown]