[physfs] PHYSFS_mountHandle and duplicate dummy filenames

[Ryan C. Gordon]

> 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.

That's a good point; this is a bug in the documentation. The filename 
needs to be unique.

(We _do_ have a FIXME for handling NULL filenames here...maybe we should 
drop that option.)

--ryan.