This adds bpf_program__attach_netkit() API to libbpf. Overall it is very
similar to tcx. The API looks as following:

  LIBBPF_API struct bpf_link *
  bpf_program__attach_netkit(const struct bpf_program *prog, int ifindex,
                             const struct bpf_netkit_opts *opts);

The struct bpf_netkit_opts is done in similar way as struct bpf_tcx_opts
for supporting bpf_mprog control parameters. The attach location for the
primary and peer device is derived from the program section "netkit/primary"
and "netkit/peer", respectively.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20231024214904.29825-4-daniel@iogearbox.netSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>