Merge branch 'libbpf: add unified bpf_prog_load() low-level API'

Andrii Nakryiko says:

====================

This patch set adds unified OPTS-based low-level bpf_prog_load() API for
loading BPF programs directly into kernel without utilizing libbpf's
bpf_object abstractions. This OPTS-based interface allows for future
extensions without breaking backwards or forward API and ABI compatibility.
Similar approach will be used for other low-level APIs that require extensive
sets of parameters, like BPF_MAP_CREATE command.

First half of the patch set adds libbpf API, cleans up internal usage of
to-be-deprecated APIs, etc. Second half cleans up and converts selftests away
from using deprecated APIs. See individual patches for more details.

v1->v2:
  - dropped exposing sys_bpf() into public API (Alexei, Daniel);
  - also dropped bpftool/cgroup.c fix for unistd.h include because it's not
    necessary due to sys_bpf() staying as is.

Cc: Hengqi Chen <hengqi.chen@gmail.com>
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>