iptables does not generate any rules from protocol:: icmp icmpv6

The iptables generator does not generate any rules from `protocol:: icmp icmpv6` but packetfilter does.

```
header {
target:: iptables INPUT DROP
target:: packetfilter test-filter
}

term permit-icmp {
protocol:: icmp icmpv6
action:: accept
}
```

Output iptables
```
# Iptables INPUT Policy
# $Id:$
# $Date:$
# $Revision:$
# inet
-P INPUT DROP
```

Output packetfilter
```
# Packetfilter test-filter Policy
# $Id:$
# $Date:$
# $Revision:$
# inet

# term permit-icmp
pass quick inet proto { icmp ipv6-icmp } from { any } to { any } keep state
```