iproute2, dhclient: Deal with v4/v6 protocols separately

When taking over DHCP services, stop the previously running dhclient
only if the protocol matches. Similarly, flush only the respective
adresses (also keep link-local v6 addresses for DHCPv6 functionality).

Change-Id: I5708c4db6f6d450536bd243a0475164b89f46f93
Signed-off-by: Ondřej Svoboda <osvoboda@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31104
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>