e.g.

	slapns -U/tmp:/run

will not unshare /tmp and /run from its parent namespace.

Without it e.g. screen does not work (it just was terminating after
right start).

It is handy to reuse slapns uid setup, because e.g. `unshare -U...` does
not allow to use newuidmap & friends, and so with just unshare it is
hard to create a user namespace where e.g. screen will work (openpty
wants to chown(:tty), gid(tty)=5)

Else screen complains:

	Cannot make directory '/run/screen': No such file or directory

Else screen complains:

(neo) (z-dev) (g.env) I have no name!@deco:/# screen
getpwuid() can't identify your account!

Before this change even ping 127.0.0.1 was failing:

	root@deco:~# ping 127.0.0.1
	connect: Network is unreachable

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).