tools: bpftool: add JSON output for `bpftool batch file FILE` command

`bpftool batch file FILE` takes FILE as an argument and executes all the
bpftool commands it finds inside (or stops if an error occurs).

To obtain a consistent JSON output, create a root JSON array, then for
each command create a new object containing two fields: one with the
command arguments, the other with the output (which is the JSON object
that the command would have produced, if called on its own).

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>