Support \ paths within filenames (Linux/Android?/etc.)

Technically, these should not be handled for disc0:/ paths, nor should case folding.

In fact, the rules seem different, since "disc0:/blah/../PSP_GAME/PARAM.SFO" fails, but "ms0:/PSP/blah/../__pspautotests_testfile.txt" passes.

But, anyway, this may fix issues with wrong slashes for savedata and homebrews on Linux (and I assume Android, Mac, etc.)

-[Unknown]