ip6+carp: revise the generation of ND6 NA

[ae]

Summary

• use ND_NA_FLAG_ROUTER flag in carp_send_na() when we work as router
• use in6addr_any as destination address for nd6_na_output(), then it will use ipv6-all-nodes multicast address.
• add in6_selectsrc_nbr() function that accepts additional argument ip6_moptions. Use this function from ND6 code to avoid cases when nd6_na_output/nd6_ns_output can not find source address for multicast destinations:

kernel: carp: 83@vlan524: BACKUP -> MASTER (master will time out)
kernel: nd6_na_output: source can't be determined: dst=ff02::1, error=51

• add some comments from RFC2461 for better understanding.
• use tlladdr argument as flags and use ND6_NA_OPT_LLA when we need to add target link-layer address option, and ND6_NA_CARP_MASTER when we know that target address is CARP master. Then we can call carp_macmatch6() if target address is CARP master and ND6 NA packet will have correct mac address.
• move blocks of code where multicast options is initialized and use it when destination address is multicast. Thus we can set M_MCAST flag and prepare ip6_moptions for correct IPv6 source address selection.