`RootFs` need to be cloneable, so the whole state can be copies instead of inherit a reference on `fork()`/`clone()`

[michalfita]

At the moment Process::fork() seem to clone the Arc<RootFs> instead of content of the RootFs itself, what leads to shared RootFs between processes, not independent file system states.

Found during work on #88 and belongs to Multithreading milestone (I can't add myself).

More details in the flags mask of clone(2), scroll down to CLONE_FS. I haven't checked yet how it's implemented in Linux Kernel.

[nuta]

I agree. We should make RootFs cloneable. Simply adding #[derive(Clone)] should work.

[nuta]

Just letting you know that according to clone(2)'s man page, mount points are controlled through another flag CLONE_NEWNS despite Kerla maintains them in RootFs. We'll need to isolate the mount_points field some day but for now, I think we don't need to support the flag unless we support running something like Docker.

[michalfita]

Yes, @nuta - all flags related to namespaces are for now excluded from my attempts, namespaces are huge topic related to contenerization.