6relayd: RFC 6204 and naming improvements

* Announce delegated prefixes using route info according to RFC 6204 L-3
* Enable hybrid stateless + stateful DHCPv6-mode as default

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36738 3c298f89-4303-0410-b956-a3cf2f4a3e73