Add ABI-related tests for ipfw(8).
The tests are handy to ensure smooth upcoming libipfw(3) introduction.
This diff adds the python framework for parsing and generating ipfw ioctls (rules only ATM).
ipfw(8) is extended with `-x` key which triggers dumping ioctls (wrapped in the debug header) to the stdout, where they can be read by the tests.
Most likely, this interface is temporary for the duration of ipfw(8) conversion to use libipfw. After the conversion, the different, yet-to-be-created interface will be used.
The current form is handy as it allows "cheap" testing without any side-effects.