Switch if_nameindex(3) to use the new NET_RT_IFNAMES sysctl to get the

Switch if_nameindex(3) to use the new NET_RT_IFNAMES sysctl to get the
list of interface names. At the same time switch if_nametoindex(3) and
if_indextoname(3) to use if_nameindex(3) instead of getifaddrs(3).
if_nameindex(3) exposes much less then getifaddrs(3) and is allowed by
pledge(2).
With and OK deraadt@