6relayd: Keep PD-downstream across RENEW/REBIND

git-svn-id: svn://svn.openwrt.org/openwrt@37009 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: 6relayd: Backport r37009

git-svn-id: svn://svn.openwrt.org/openwrt@37010 3c298f89-4303-0410-b956-a3cf2f4a3e73
6relayd: fix lease-update behaviour for clients without reconfigure

git-svn-id: svn://svn.openwrt.org/openwrt@37011 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: 6relayd: backport r37011

git-svn-id: svn://svn.openwrt.org/openwrt@37012 3c298f89-4303-0410-b956-a3cf2f4a3e73