Merge branch 'libbpf-versioning-doc'

Andrey Ignatov says:

====================
This patch set adds ABI versioning and documentation to libbpf.

Patch 1 renames btf_get_from_id to btf__get_from_id to follow naming
convention.
Patch 2 adds version script and has more details on ABI versioning.
Patch 3 adds simple check that all global symbols are versioned.
Patch 4 documents a few aspects of libbpf API and ABI in dev process.

v1->v2:
* add patch from Martin KaFai Lau <kafai@fb.com> to rename btf_get_from_id;
* add documentation for libbpf API and ABI.
====================

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