fix zsync cannot run properly in Kindle due to firewall

Shouldn't we do some check whether rules were already added to iptables in a previous run of KOReader? Otherwise the rules will be added multiple times:
```
[root@kindle root]# iptables-save 
...
-A INPUT -i wlan0 -p udp -m udp --dport 5670 -j ACCEPT 
-A INPUT -i wlan0 -p udp -m udp --dport 5670 -j ACCEPT 
...
```