Use iptables-save and parse the output

`iptables -S` didn't work on older OSs, so the tests have been adapted
for that.

There was one test for the NAT table that I'm not sure what the purpose
was, since it seemed to be testing munge instead. I edited it to get it
to pass.
Merge pull request #311 from hunner/fix_nobody

Use iptables-save and parse the output