libbpf-tools/klockstat: no more rlimit bumping + add comment

libbpf-tools/klockstat: no more rlimit bumping + add comment

The PR adding the libbpf-tools port of klockstat was sitting in a
mergeable state for some time. Meanwhile, libbpf stopped exposing
rlimit_memlock bumping API and now does the rlimit bump automatically if
necessary. So remove the bump_rlimit_memlock call and set libbpf strict
mode for this tool.

Also, add a comment (from @brho's PR summary in #3688) detailing the
differences in default behavior between the libbpf-tools and bcc-python
versions.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>