The kernel now provides ids for peer netns. This patch implements a new command
'set' to assign an id.
When netns are listed, if an id is assigned, it is now displayed.

Example:
 $ ip netns add foo
 $ ip netns set foo 1
 $ ip netns
 foo (id: 1)
 init_net
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>