Merge branch 'libbpf: allow users to set kprobe/uprobe attach mode'

Merge branch 'libbpf: allow users to set kprobe/uprobe attach mode'

Menglong Dong says:

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

From: Menglong Dong <imagedong@tencent.com>

By default, libbpf will attach the kprobe/uprobe BPF program in the
latest mode that supported by kernel. In this series, we add the support
to let users manually attach kprobe/uprobe in legacy/perf/link mode in
the 1th patch.

And in the 2th patch, we split the testing 'attach_probe' into multi
subtests, as Andrii suggested.

In the 3th patch, we add the testings for loading kprobe/uprobe in
different mode.

Changes since v3:
- rename eBPF to BPF in the doc
- use OPTS_GET() to get the value of 'force_ioctl_attach'
- error out on attach mode is not supported
- use test_attach_probe_manual__open_and_load() directly

Changes since v2:
- fix the typo in the 2th patch

Changes since v1:
- some small changes in the 1th patch, as Andrii suggested
- split 'attach_probe' into multi subtests
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>