src: support zone set statement with optional direction

src: support zone set statement with optional direction

nft automatically understands 'ct zone set 1' but when a direction is
specified too we get a parser error since they are currently only
allowed for plain ct expressions.

This permits the existing syntax ('ct original zone') for all tokens with
an optional direction also for set statements.

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