routing: Report the nexthop family of nexthop group members

[w453y.me_gmail.com]

Summary

The members of an IPv4 nexthop group can have IPv6 gateways. Their
nh_idx is from the IPv6 nexthop table, so netstat -On showed the IPv4
nexthop with the same index, or nothing.

Append two containers to the nexthop group sysctl entry with the neighbor
family of each control plane and dataplane nexthop. Existing consumers
read the first two containers and step by rtm_msglen, so they are not
affected.

Use them in netstat to look up IPv6 nexthops in the IPv6 table, to count
the slots of nexthops with the same index in both tables, and widen the
gateway column for such groups.

Signed-off-by: Abdul Wasey <[e-mail]>

Test Plan

• A group with IPv4 and IPv6 members, including an IPv4 and an IPv6 nexthop with the same index: netstat -On shows the right gateway and slots for each member (before: the IPv4 nexthop with the same index, or nothing, and the slots of both added together).
• Old netstat on the new kernel prints the same output as before, and new netstat on the old kernel falls back to the old output.
• New test test_rtm_multipath4_mixed_nhgrp_dump.
• kyua sys/net/routing: 86/87 pass (same known failure), sys/netlink: 116/117 pass, 1 skipped.