Work around intermittent iptables exit status 4 errors

See also: ukanth/afwall#179

Various users have reported intermittent problems in which iptables returns exit status 4 (iptables: Try again).  This can happen if the Android system is updating the "filter" table (any chain) at the same time we are.  These updates generally aren't modifying the same chain or rules, so they shouldn't conflict with each other in any other way.

The problem can be reproduced by applying the AFWall rules while running this loop in another root shell:

```
    iptables -N foobar
    while :; do iptables -A foobar -d 1.2.3.4 -j ACCEPT ; \
        iptables -D foobar -d 1.2.3.4 -j ACCEPT ; done
```

We will retry these transactions up to 10 times.