rootless: use a single user namespace

simplify the rootless implementation to use a single user namespace
for all the running containers.

This makes the rootless implementation behave more like root Podman,
where each container is created in the host environment.

There are multiple advantages to it: 1) much simpler implementation as
there is only one namespace to join.  2) we can join namespaces owned
by different containers.  3) commands like ps won't be limited to what
container they can access as previously we either had access to the
storage from a new namespace or access to /proc when running from the
host.  4) rootless varlink works.  5) there are only two ways to enter
in a namespace, either by creating a new one if no containers are
running or joining the existing one from any container.

Containers created by older Podman versions must be restarted.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

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