Fix ip_forwarding (issue #150 and 151)

I want to use vuurmuur 0.8rc1, so I retrieve the latest code from GitHub and compile everything. But there was one important bug that was preventing me of using this latest version. The flag ip_forwarding was disabled.

I reviewed the code and the function post_rules (found in createrule.c) is called twice. Once for IPv4 and once for IPv6. I use only IPv4 rules. While debugging I found out that the first time ip_forward was set and the second time it was reset. There was no check in the function if the function call was made for IPv4 or IPv6.

This solution works for me.