When attaching to only some namespaces of the container but not the mount
namespace, the contents of /sys and /proc of the host system do not properly
reflect the context of the container's pid and/or network namespaces, and
possibly others.

The introduced -R option adds the possibility to additionally unshare the
mount namespace (when it is not being attached) and remount /sys and /proc
in order for those filesystems to properly reflect the container's context
even when only attaching to some of the namespaces.
Signed-off-by: Christian Seiler <christian@iwakd.de>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>