[physfs] Getting every instance of overlapping files

[Sik the hedgehog]

Two options:

- Mount them in separate directories and just deal with the fact (I
ended up doing this to prevent mods in my game from overwriting
configuration and savegame files).

- If the data is going to permanently stay in memory (i.e. you won't
need to read the files after initially mounted), then load the files
the moment you mount them (before mounting the subsequent archives).
The obvious upshot is that if you want you can unmount them after
loading (since the data is in RAM already).