src: prepare for future ct timeout policy support

src: prepare for future ct timeout policy support

Change all places that expect ct helper tokens (ct helper configuration)
to CT HELPER.  ct_obj_kind is removed.

When we add ct timeout support, we will add a new ct_timeout_block,
plus extra rules.  We won't extend ct_block, it prevents the parser
from detecting bogus syntax that only makes sense for ct helper but
not for something else for instance.

ct_block should be renamed to ct_helper_block, will be done in
followup patch.

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