ovs-brcompatd: Use rtnetlink_link_notifier instead of open-coding it.

ovs-brcompatd: Use rtnetlink_link_notifier instead of open-coding it.

ovs-brcompatd has always had its own code to listen on an RTNL socket, but
I don't see any reason for it.  This commit rips it out in favor of
rtnetlink_link_notifier.

This change looks fairly big but a lot of it boils down to changing the
indentation level of rtnl_recv_update().