packets: add compose_nd_ra

packets: add compose_nd_ra

This patch introduces methods to compose a Router Advertisement (RA) packet,
introduces flags for RA. RA packet composed structures against specification
in RFC4861.

Caller can use compse_nd_ra_with_sll_mtu_opts to compose a RA packet with
Source Link-layer Address Option and MTU Option.

Caller can use packet_put_ra_prefix_opt to append a Prefix Information Option
to a RA packet.

Signed-off-by: Zongkai LI <zealokii@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>