JSON: Add metainfo object to all output

JSON: Add metainfo object to all output

Right now this object merely contains the nftables version and release
name as well as a JSON schema version, but it could be extended
arbitrarily. In the future, this will also allow for non-compatible
schema changes should the need for this arise.

Adjust the parser to accept metainfo objects and make it verify
json_schema_version to be less than or equal to the one hard-coded in
the library.

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