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
ensure INPUT, OUTPUT and FORWARD firewallchain are not autorequired when table is :filter (to allow default drop policy at the end of rules)
Add a .fixtures.yml so spec_helper can manage the tests.
Merge pull request #241 from apenney/remove

Add a .fixtures.yml so spec_helper can manage the tests.
Prepare 0.4.2 release.
Merge pull request #242 from apenney/042-release

Prepare 0.4.2 release.
Fix pre/post errors
Merge pull request #247 from edmundcraske/patch-1

Fix pre/post errors
Merge pull request #208 from georgkoester/hop_limit2

Add support for IPv6 hop limiting
Merge pull request #240 from doc75/dev_drop_policy

No firewallchain autorequire for INPUT, OUTPUT and FORWARD when table is :filter to enable DROP policy without blocking