netns: add subcommand to attach an existing network namespace

netns: add subcommand to attach an existing network namespace

ip tracks namespaces with dummy files in /var/run/netns/, but can't see
namespaces created with other tools.
Creating the dummy file and bind mounting the correct procfs entry will
make ip aware of that namespace.
Add an ip netns subcommand to automate this task.

Signed-off-by: Matteo Croce <mcroce@redhat.com>
Reviewed-by: Andrea Claudi <aclaudi@redhat.com>
Tested-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: David Ahern <dsahern@gmail.com>