Add support for Linux network namespaces

[edugrasa]

When using network resources from the Linux kernel (e.g. network interfaces in the shim-eth or shim-wifi, or TCP/UDP ports in the shim-tcp-udp), provide support to choose a specific network namespace (right now only the default one is used).

[vmaffione]

I did that some time ago for for the light implementation. It may be interesting to have a look at that:
https://github.com/rlite/rlite/blob/master/kernel/ctrl-dev.c#L124-L127

[edugrasa]

Thanks @vmaffione, will definitely take a look!