[NEIGH]: Netlink notifications

Currently neighbour event notifications are limited to update
notifications and only sent if the ARP daemon is enabled. This
patch extends the existing notification code by also reporting
neighbours being removed due to gc or administratively and
removes the dependency on the ARP daemon. This allows to keep
track of neighbour states without periodically fetching the
complete neighbour table.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>