lxccontainer: fix enter_net_ns helper to work when netns is inherited

lxccontainer: fix enter_net_ns helper to work when netns is inherited

If a network namespace is shared by setting lxc.namespace.share.net and
the container is unprivileged, then the network namespace should be
entered before entering the user namespace. However, if an unprivileged
user started a container, then the network namespace should be entered
after entering the user namespace. To solve this, we try to enter the
network namespace before entering the user namespace. If it did not
succeed, it will be tried again inside the uder namespace.

Signed-off-by: Jef Steelant <jef.steelant_ext@softathome.com>