Import nd6_output_ifp routing function from FreeBSD

Hi tech-net/kern

I am currently working on policy based routing (based on outgoing interface)

there’ a previously written code for ipv6 which I’ve taken from Jonathan A. Kollasch long time ago. 
and he used nd6_output function which appears to be removed from netbsd. 

+ if (IN6_IS_SCOPE_EMBEDDABLE(&dst.v6.sin6_addr))
+ dst.v6.sin6_addr.s6_addr16[1] = htons(ifp->if_index);
+ 
+ if (m->m_pkthdr.len <= ifp->if_mtu) {
+ error = nd6_output(ifp, ifp, m, &dst.v6, NULL);
+ } else {
+ in6_ifstat_inc(ifp, ifs6_in_toobig);
+ icmp6_error(m, ICMP6_PACKET_TOO_BIG, 0, ifp->if_mtu);
+ }

freeBSD now uses nd6_output_ifp for a similar job. and i want to import it to netbsd so we can use it for that.

https://github.com/freebsd/freebsd-src/blob/main/sys/netinet6/nd6.c#L2184

Emmanuel