data_reg: fix verdict format approach

data_reg: fix verdict format approach

Patrick reports that the XML/JSON formats of the data_reg object
are not accuarate.

This patch updates these formats, so they are now as follow:

 * <data_reg type=value> with raw data (this doesn't change).
 * <data_reg type=verdict> with a concrete verdict (eg drop accept) and an
  optional <chain>, with destination.

In XML:
	<data_reg type="verdict">
		<verdict>goto</verdict>
		<chain>output</chain>
	</data_reg>

In JSON:
	"data_reg" : {
		"type" : "verdict",
		"verdict" : "goto"
		"chain" : "output",
	}

The default output format is updated to reflect these changes (minor collateral
thing).

When parsing set_elems, to know if we need to add the NFT_SET_ELEM_ATTR_CHAIN
flag, a basic check for the chain not being NULL is done, instead of evaluating
if the result of the parsing was DATA_CHAIN. The DATA_CHAIN symbol is no longer
used in the data_reg XML/JSON parsing zone.

While at it, I updated the error reporting stuff regarding data_reg/verdict, in
order to leave a consistent state in the library.

A JSON testfile is updated as well.

Reported-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>