mxml: add error reference of the top node

We know the top node we are building. Let the user also know it.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
set_elem: add json parsing to API

Add missing support in the API function to parse a JSON set_elem.

I've renamed the main JSON parsing function to prevent clashing.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>