Merge of more plw fixes

This resolves an odd issue where arp configuration failed if pre-existing arp config existed. More interestingly, it has a candidate fix for the crash / spin in python-iptables.
This structure is weird. It's so weird, in fact, that I missed the fact that you create one more match for IPv6 than you do for IPv4 until I rewrote the code! Consider replacing with:

```python
rules = (iptc.Rule(), iptc.Rule6())
for rule in rules:
    rule.create_match('conntrack')
    rule.create_match('tcp')
    rule.create_match('udp')
    rule.create_match('mac')
    rule.create_match('physdev')

# The IPv6 rule also needs to catch icmpv6
rules[1].create_match('icmp6')
```