Make same-dir rename() a default

The cvmfs client currently uses rename across directories in its cache space. That can lead to a deadlock in the context of overlayfs mounts. It seems that this deadlock goes away if the workaround is used to create the temporary files in the folder of their final destination. If this turns out to be true, this workaround should be the new default. It would avoid special treatment of cache directories on BeeGFS. The cvmfs_fsck binary might need to be adjusted to not complain about temporary files that are not in /txn.