Two commands are added:
 - ip netns list-id
 - ip monitor nsid

A cache is also added to remember the association between the iproute2 netns
name (from /var/run/netns/) and the nsid.
To avoid interfering with the rth socket, a new rtnl socket (rtnsh) is used to
get nsid (we may send rtnl request during listing on rth).

Example:
$ ip netns list-id
nsid 0 (iproute2 netns name: foo)
$ ip monitor nsid
Deleted nsid 0 (iproute2 netns name: foo)
nsid 16 (iproute2 netns name: bar)
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>