Enable network namespace sharing in lxc-start

Enable network namespace sharing in lxc-start

Right now lxc-start always does one of two things: it creates
a new namespace or inherits it from the parent environment.
This patch adds a third option: share a namespace with another
container (actually: a process).

In some situations this is handy. For example by sharing a network
namespace it is possible to migrate services between containers
without (or with little) downtime.

This patch creates an infrastructure for inheriting any type
of namespace, but only the network namespace is supported for now.