Block IPv6 in proxy+auto filtering mode

[Revertron]

Since IPv6 can't be routed to our proxy in root-proxy mode as we see in #580 and we have a trouble with filtering IPv6 when there is IPv6 network on Wi-Fi and Mobile as in #142 we are introducing blocking IPv6 traffic.

We must add low-level settings for user to be able to control this behavior - pref.net.block_ipv6 and pref.net.always_filter_ipv6.

[ameshkov]

Name it pref.proxy.block.ipv6, true by default.

[Revertron]

Okay, I've made blocking of only external connections. Added the option to low-level group.

[Revertron]

Now we are blocking all IPv6 TCP connections and UDP (QUICK) only for Google Chrome, but passing through all other UDP packets because users will want to use some UDP-based VPN on arbitrary ports.