tools: bpftool: add a command to dump the trace pipe

BPF programs can use the bpf_trace_printk() helper to print debug
information into the trace pipe. Add a subcommand
"bpftool prog tracelog" to simply dump this pipe to the console.

This is for a good part copied from iproute2, where the feature is
available with "tc exec bpf dbg". Changes include dumping pipe content
to stdout instead of stderr and adding JSON support (content is dumped
as an array of strings, one per line read from the pipe). This version
is dual-licensed, with Daniel's permission.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
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>

tools/bpf/bpftool/bash-completion/bpftool

@@ -398,10 +398,13 @@ _bpftool()
                             ;;
                     esac
                     ;;
+                tracelog)
+                    return 0
+                    ;;
                 *)
                     [[ $prev == $object ]] && \
                         COMPREPLY=( $( compgen -W 'dump help pin attach detach load \
-                            show list' -- "$cur" ) )
+                            show list tracelog' -- "$cur" ) )
                     ;;
             esac
             ;;