netfilter: nf_nat: add nat hook register functions to nf_nat

This adds the infrastructure to register nat hooks with the nat core
instead of the netfilter core.

nat hooks are used to configure nat bindings.  Such hooks are registered
from ip(6)table_nat or by the nftables core when a nat chain is added.

After next patch, nat hooks will be registered with nf_nat instead of
netfilter core.  This allows to use many nat lookup functions at the
same time while doing the real packet rewrite (nat transformation) in
one place.

This change doesn't convert the intended users yet to ease review.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>