Add support for attaching an eBPF object by file descriptor.

The iptables binary can be called with a path to an elf object or a
pinned bpf object. Also pass the mode and path to the kernel to be
able to return it later for iptables dump and save.
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>