Introduce a new XML-based iptables parser.
The old `iptables_parse()` way relies on regex parsing the output of the `iptables` output, which of course is not very reliable. In Jessie the `ip6tables` output has changed and differs (if there are no options the "opt" column will be empty, and not have a "--"). The new parser fixes this. The goal will be to convert all `iptables_parse()` instances to the new parser. Will-fix: #9704
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