libbpf: Support guessing post_bind{4,6} progs

libbpf can guess prog type and expected attach type based on section
name. Add hints for "cgroup/post_bind4" and "cgroup/post_bind6" section
names.

Existing "cgroup/sock" is not changed, i.e. expected_attach_type for it
is not set to `BPF_CGROUP_INET_SOCK_CREATE`, for backward compatibility.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>