FR: Avoid enabling IP forwarding on the host

[ccdunder]

Enabling IP forwarding increases the attack service of any machine. I don't believe it's necessary.

@Maroka-chan do you have any reservations about merging such a change?

(Excellent work, btw)

[Maroka-chan]

I've also had concerns about it, but IP forwarding seems to be necessary to pre-route packets to the networking namespace :(
I haven't found a way to only allow forwarding to the netns.. It'd be interesting to see what Docker does.
I'd definitely be open to a solution that doesn't require IP forwarding :)