netfilter: core: export raw versions of add/delete hook functions

This will allow the nat core to reuse the nf_hook infrastructure
to maintain nat lookup functions.

The raw versions don't assume a particular hook location, the
functions get added/deleted from the hook blob that is passed to the
functions.

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