netconf: add proxy-arp support

Add support to netconf to show changes to proxy-arp status on a per
interface basis via netlink in a manner similar to forwarding
and reverse path state.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>