Given a container "foo", that maps user id $UID to container user, using
user namespaces, this NSS module extenstion will now map the $UID to a
name "vu-foo-$TUID" for the translated UID $UID.
Similar, userns groups are mapped to "vg-foo-$TGID" for translated GIDs
of $GID.
This simple change should make userns users more discoverable. Also,
given that many tools like "adduser" check NSS before allocating a UID,
should lower the chance of UID range conflicts between tools.
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