tools: bpftool: introduce --json and --pretty options

These two options can be used to ask for a JSON output (--j or -json),
and to make this JSON human-readable (-p or --pretty).

A json_writer object is created when JSON is required, and will be used
in follow-up commits to produce JSON output.

Note that --pretty implies --json.

Update for the manual pages and interactive help messages comes in a
later patch of the series.

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>