The hypothesis for this patch is that the operating system has not actually
finished writing the file to disk when moving the tmp file into place. The
move operation is atomic, but the write is not, even when flushed or using
unbuffered IO. So we reorder these operations, make the old save .bak
atomically and write the new save in place.

I doubt saving this backup will actually be useful given the frequency of
saves in the game, but at the very least it leaves _some_ backup in place in
the event of failure.