Merge branch 'libbpf: support custom SEC() handlers'

Andrii Nakryiko says:

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

Add ability for user applications and libraries to register custom BPF program
SEC() handlers. See patch #2 for examples where this is useful.

Patch #1 does some preliminary refactoring to allow exponsing program
init, preload, and attach callbacks as public API. It also establishes
a protocol to allow optional auto-attach behavior. This will also help the
case of sometimes auto-attachable uprobes.

v4->v5:
  - API documentation improvements (Daniel);
v3->v4:
  - init_fn -> prog_setup_fn, preload_fn -> prog_prepare_load_fn (Alexei);
v2->v3:
  - moved callbacks and cookie into OPTS struct (Alan);
  - added more test scenarios (Alan);
  - address most of Alan's feedback, but kept API name;
v1->v2:
  - resubmitting due to git send-email screw up.

Cc: Alan Maguire <alan.maguire@oracle.com>
====================

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