src: allow to specify the default policy for base chains

src: allow to specify the default policy for base chains

The new syntax is:

 nft add chain filter input { hook input type filter priority 0\; policy accept\; }

but the previous syntax is still allowed:

 nft add chain filter input { hook input type filter priority 0\; }

this assumes default policy to accept.

If the base chain already exists, you can update the policy via:

 nft add chain filter input { policy drop\; }

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>