6relayd: Work around Windows DHCPv6 quirks
Announcing deprecated addresses breaks Windows clients

git-svn-id: svn://svn.openwrt.org/openwrt@36815 3c298f89-4303-0410-b956-a3cf2f4a3e73
AA: 6relayd: backport r36815

git-svn-id: svn://svn.openwrt.org/openwrt@36816 3c298f89-4303-0410-b956-a3cf2f4a3e73