Instead remap user IDs that we have to continous [0, ...] IDs in the target namespace.

We need to be able to use e.g. tty group from inside (gid=5), because
glibc.openpty wants to chwon files in /dev/pts to that group.

See next patch for /dev/pts setup.

Else for regular user we are doomed to always have only single

	X -> UID-in-parent

mapping.

See details in comments inside.

TODO handle case when we have CAP_SETUID already (slapns inside slapns).