Setting the transit namespace at runtime

[Julian Orth]

Hi,

After receiving some positive feedback on IRC, I've gone ahead and implemented
this. You can see the code here:

https://github.com/mahkoh/wireguard/commits/transit-namespace

You can test it as follows:

* Create a new netns: ip netns add test
* Enter it: ip netns exec test bash
* Use wg-quick to create a wireguard device
* Try to connect anywhere: It doesn't work
* Set the transit namespace to the init namespace:
    wg set wg0 transit-net /proc/1/ns/net
* Try to connect anywhere: It works

I haven't written any documentation yet but I hope that the commits are clear
enough.

Julian