src: rename the parameter tag to node_name in jansson function

I have changed this parameter for having consistence with the
xml helper function. This patch is a cleanup.

Signed-off-by: Alvaro Neira Ayuso <alvaroneay@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
src: new error reporting approach for XML/JSON parsers

I have added a new structure for reporting some errors in parser
that we can't cover with errno.

In this patch, we have three errors that we can't cover with errno:

NFT_PARSE_EBADINPUT : Bad XML/JSON format in the input
NFT_PARSE_EMISSINGNODE : Missing node in our input
NFT_PARSE_EBADTYPE : Wrong type value in a node

Signed-off-by: Alvaro Neira Ayuso <alvaroneay@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>