DHCPv6 enhancements to support RFC6177

New CLI commands are introduced to configure preferred prefix length and sub-prefix length in clients. A delegating router (DHCPv6 server) is provided with IPv6 prefixes and a requesting router (DHCPv6 client) requests one or more prefixes from the delegating router. When the client receives a valid DHCPv6 block it must then delegate to all active interfaces using a sub-prefix delegation.