yet another version of fuidshift
the only reason why this exists is that it was faster to GPT^H^H^Hwrite it myself than to chase where did fuidshift binary go
it was/is used to fix botched id mapping in LXD, e.g.
run without parameters to see the syntax
./fuidshift.py
Usage: [export DEBUG=yes]; ./fuidshift.py <directory> <offset>
Example (LXD shift down): sudo ./fuidshift.py /var/lib/lxd/containers/mycontainer/rootfs -1000000