rtnetlink: Extend rtnetlink to support RTNLGRP_IPV4_IFADDR and

rtnetlink: Extend rtnetlink to support RTNLGRP_IPV4_IFADDR and
RTNLGRP_IPV6_IFADDR.

This commit renames the rtnetlink-link.{c,h} to rtnetlink.{c,h}
and extends the module to support RTNLGRP_IPV4_IFADDR and
RTNLGRP_IPV4_IFADDR multicast groups.  A later patch will start
using this module to react to interface address changes.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>