The former is meant to send PhysicsFS-buffered data to the PHYSFS_Io's
implementation, the latter is meant to tell the OS to definitely make sure the
data is safely written to disk (or at least, that's what it does in practice).

This was making PHYSFS_setBuffer()'d handles _slower_, since they would end
up blocking whenever the buffer was full until the data made the full trip to
physical media, instead of just letting the OS do its own buffering.

Now we still PHYSFS_Io::flush() on PHYSFS_close(), like this has always
worked. That might also be overkill, but that remains a historical artifact
of trying to keep the underlying file handle usable if pending writes fail
for possibly-recoverable reasons (which isn't guaranteed if you just close()
it, at least as far as I remember).