parser_json: Fix for ineffective family value checks

parser_json: Fix for ineffective family value checks

Since handle->family is unsigned, checking for value < 0 never yields
true. Overcome this by changing parse_family() to return an error code
and write the parsed family value into a pointer passed as parameter.

The above change required a bit more cleanup to avoid passing pointers
to signed variables to the function. Also leverage json_parse_family() a
bit more to reduce code side.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>