ct: allow numeric conntrack labels

ct: allow numeric conntrack labels

When dumping labels in rule list we try to print a symbolic name.
If we don't find one, we print the bit number instead.

This changes nft to also allow use of the number instead of a name
when adding ct label rules so that such dumps can also be restored
again.

This is similar to other cases, e.g. skuid root vs skuid 0 which
are both valid.

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