Make restricted-network prevent use of netfilter

[KellerFuchs]

I suggest netfilter to be affected by restricted-network because it is a traditionally-privileged interface to the kernel, and the security impact of letting untrusted users run their own netfilter scripts (even on their own network namespace) is non-obvious.

Is that OK, or should I introduce a separate config flag for this?

[KellerFuchs]

PS: Since users can only use --net=none when restricted-network is set, there is no feature-loss with this change.

[netblue30]

Merged, thanks.

[KellerFuchs]

Thanks a bunch.