nft: Split nft core to become family independant

This makes nft core code independant from the family. Each family needs
to implement and provide a struct nft_family_ops {}.

This split will ease the future support of bridge and arp rules manipulations.

[ updated header files and rebased upon the current tree --pablo ]

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
8 files changed