6relayd: Keep PD-downstream across RENEW/REBIND

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37009 3c298f89-4303-0410-b956-a3cf2f4a3e73
6relayd: fix lease-update behaviour for clients without reconfigure

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37011 3c298f89-4303-0410-b956-a3cf2f4a3e73