src: enable json echo output when reading native syntax

src: enable json echo output when reading native syntax

This patch fixes a bug in which nft did not print any output when
specifying --echo and --json and reading nft native syntax.

This patch respects behavior when input is json, in which the output
would be the identical input plus the handles.

Adds a json_echo member inside struct nft_ctx to build and store the json object
containing the json command objects, the object is built using a mock
monitor to reuse monitor json code. This json object is only used when
we are sure we have not read json from input.

[ added json_alloc_echo() to compile without json support --pablo ]

Fixes: https://bugzilla.netfilter.org/show_bug.cgi?id=1446
Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Tested-by: Eric Garver <eric@garver.life>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>