netfilter: conntrack: add mnemonics for sysctl table

Its a bit hard to see what table[3] really lines up with, so add
human-readable mnemonics and use them for initialisation.

This makes it easier to see e.g. which sysctls are not exported to
unprivileged userns.

objdiff shows no changes.

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