[physfs] PHYSFS_mountHandle and duplicate dummy filenames

[Aleksi Nurmi]

The documentation says:

"(filename) is only used here to optimize archiver selection (if you name it
XXXXX.zip, we might try the ZIP archiver first, for example). It doesn't
need to refer to a real file at all, and can even be NULL. If the filename
isn't helpful, the system will try every archiver until one works or none
of them do."

I called the function two times, but passed "X.zip" as the filename in
both calls. This caused doMount in physfs.c to fail, because it checks
if a search path already exists.

I didn't attempt a fix, because maybe it's the documentation that
needs to change - if you pass two identical filenames, how can you
unmount only one of them?