iptables-compat: fix chain policy reset with iptables -L -n

Initialize built-in tables/chains if they don't exists, otherwise
simply skip.

This avoids the chain policy reset to NF_ACCEPT by when you call
iptables -L -n.

Reported-by: Ana Rey <anarey@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Ana Rey <anarey@gmail.com>
iptables-compat: statify unused built-in table/chain functions

The functions that allows you to create built-in table and chains are
required out of the scope of nft.c

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
iptables-compat: assume chain policy NF_ACCEPT when creating built-in chains

Newly created (emulated) xt built-in chain have to use NF_ACCEPT. Remove
extra unused chain parameter and rename nft_chain_builtin_init to
nft_xt_builtin_init too.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
iptables-compat: fix empty chains after first invocation of iptables-compat -L

 # iptables-compat -L
 # iptables-compat -L
 Chain INPUT (policy ACCEPT)
 target     prot opt source               destination

 Chain FORWARD (policy ACCEPT)
 target     prot opt source               destination

 Chain OUTPUT (policy ACCEPT)
 target     prot opt source               destination

Note that the second (and follow up) invocations after the first one
display the chains.

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