linux: enable additional netfilter options.

linux.config is currently a bit too sparing with enabled netfilter options, which leads to issues with iptables. A simple test case:

$ iptables -A INPUT -p tcp --dport 3306 -m state --state NEW -s localnet/24 -j ACCEPT

That will produce potentially two different error messages if kernel options are not right. One will be complaining that tables are not available, which means you can't add any rules. The other will be complaining that it doesn't understand 'localnet/24' and the rule will fail to add. 

I believe this to be the correct-enough solution, enabling 14 options: 

* CONFIG_NETFILTER_XTABLES and  CONFIG_NETFILTER_XT_MATCH_STATE are required. CONFIG_IP6_NF* are needed for IPv6 support as well.

* NF_CONNTRACK*'s enabled protocols mirror NF_NAT's exactly. I have already broken iptables by attempting to delete every protocol from there but SNMP, PPTP and IPV4/V6. However, there may (or may not!) be room for improvement here. This block does need to exist in some form and due to it mirroring the existing NF_NAT options, I consider it correct.

The .gitignore adjustment has made working in the tree I intent to push from more pleasant and I cannot think of a reason why one would want a user-edited config or super_chroot binary to wind up in-tree.