Add ipv6 fragmentation matchers and generify known_boolean handling.

Adds tests for ipv6, too.

ip6tables handles fragmentation differently. There's a special
module and a couple of matchers which are all needed to
implement a stateless firewall correctly.

known_boolean handling with etc has been generified.
The known_boolean functionality was partly tailored
to the :socket feature.
Add support for hop limit
Merge pull request #208 from georgkoester/hop_limit2

Add support for IPv6 hop limiting