Merge branch 'move_BPF_PROG_to_libbpf'

Merge branch 'move_BPF_PROG_to_libbpf'

Andrii Nakryiko says:

====================
Move BPF_PROG, BPF_KPROBE, and BPF_KRETPROBE helper macros from private
selftests helpers to public libbpf ones. These helpers are extremely helpful
for writing tracing BPF applications and have been requested to be exposed for
easy use (e.g., [0]).

As part of this move, fix up BPF_KRETPROBE to not allow for capturing input
arguments (as it's unreliable and they will be often clobbered). Also, add
vmlinux.h header guard to allow multi-time inclusion, if necessary; but also
to let PT_REGS_PARM do proper detection of struct pt_regs field names on x86
arch. See relevant patches for more details.

  [0] https://github.com/iovisor/bcc/pull/2778#issue-381642907
====================

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