* 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.