"random" is a new IPv6 addrgenmode, enabling "stable_secret" type
addresses with an auto-generated secret.

$ ip link set eth0 addrgenmode random

$ ip -d link show dev eth0
2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether 00:21:86:a3:25:7d brd ff:ff:ff:ff:ff:ff promiscuity 0 addrgenmode random

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>