The "-d" option is used to require all logs available for bpftool. So
far it meant telling libbpf to print even debug-level information. But
there is another source of info that can be made more verbose: when we
attemt to load programs with bpftool, we can pass a log_level parameter
to the verifier in order to control the amount of information that is
printed to the console.

Reuse the "-d" option to print all information the verifier can tell. At
this time, this means logs related to BPF_LOG_LEVEL1, BPF_LOG_LEVEL2 and
BPF_LOG_STATS. As mentioned in the discussion on the first version of
this set, these macros are internal to the kernel
(include/linux/bpf_verifier.h) and are not meant to be part of the
stable user API, therefore we simply use the related constants to print
whatever we can at this time, without trying to tell users what is
log_level1 or what is statistics.

Verifier logs are only used when loading programs for now (In the
future: for loading BTF objects with bpftool? Although libbpf does not
currently offer to print verifier info at debug level if no error
occurred when loading BTF objects), so bpftool.rst and bpftool-prog.rst
are the only man pages to get the update.

v3:
- Add details on log level and BTF loading at the end of commit log.

v2:
- Remove the possibility to select the log levels to use (v1 offered a
  combination of "log_level1", "log_level2" and "stats").
- The macros from kernel header bpf_verifier.h are not used (and
  therefore not moved to UAPI header).
- In v1 this was a distinct option, but is now merged in the only "-d"
  switch to activate libbpf and verifier debug-level logs all at the
  same time.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>