rtnl: add a new type of msg to advertise protocol configuration

A new type is added to allow userland to monitor protocol configuration, like
IPv4 or IPv6.
For example, monitoring the state of the forwarding status of an interface of
the system.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>