[physfs] How to check if some files got overriden?

[Sik the hedgehog]

I'm using PhysicsFS mounting to implement mods in my game (where a mod
is just an archive that overrides those in the game). I need a way to
check if some of those get altered when I add a mod though (to disable
saving, otherwise the savegame could become invalid). More
specifically, I need to see if the archive:

- Contains system/scenes
- Contains system/tweaks.ini
- Contains levels/*.sol

How can I check this? I would do the check every time I mount a mod
(not after mounting all of them), if this helps.