lxc-start takes a pre-created network namespace path

lxc-start now takes a network namespace path that is created before
the container creation. The behavior is similar to --share-net, except
that it does not require a container name or pid. Instead it takes the
path to the namespace and gets the fd out of that.

Signed-off-by: Abin Shahab <ashahab@altiscale.com>