iproute2: add support for IFLA_NET_NS_PID in ip link

iproute2: add support for IFLA_NET_NS_PID in ip link

Hi Stephen,

I resend you this patch once more. This time I updated the documentation
too (may be that was the reason why you didn't take it before?).

Please tell me if there are other things missing in this patch
It applies on iproute2 git tree.

Regards,
Benjamin

Description:
------------

This patch adds support for the IFLA_NET_NS_PID type. It is used to
move network devices between network namespaces.

The syntax is:
ip link set DEVICE netns PID

PID is the pid of a process in the target network namespace.

(Daniel Lezcano is the original author).

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: Stephen Hemminger <stephen.hemminger@vyatta.com>