odhcpd: improve DHCPv6-PD detection

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42160 3c298f89-4303-0410-b956-a3cf2f4a3e73
netifd: assign ipv6-prefixes with length <64 with /64 on-link routes

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42161 3c298f89-4303-0410-b956-a3cf2f4a3e73