This patch allows the user to select any list of namespaces (network, pid,
mount, uts, ipc, user) that lxc-attach should use when attaching to the
container; all other namespaces will not be attached to.

This allows the user to for example attach to just the network namespace and
use the host's (and not the container's) network tools to reconfigure the
network of the container.
Signed-off-by: Christian Seiler <christian@iwakd.de>
Cc: Daniel Lezcano <daniel.lezcano@free.fr>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>