ifa_data member of every AF_LINK struct ifaddrs points at the
corresponding struct if_data. In ifconfig(8), do not try to suppress
duplicate AF_LINK ifaddrs by checking for a NULL ifa_data.
Don't copy out two AF_LINK struct ifaddrs for each active link-layer
address. getifaddrs(3) used to copy out one ifaddrs for the kernel's
RTM_IFINFO message, and one more for the kernel's RTM_NEWADDR message.
I suppress the first duplicate with a highly conservative change that
wastes a little bit of ifaddrs storage. The storage is not leaked.
46db29bcf1