Links v2: Execin & Ethernet move

This is a part of linksv2. This provides two sets of coupled functionality. `execin` provides a way to execute operations within a namespace (via libcontainer) and `MoveEthernetDevice` is hopefully self-explanatory.

I don't currently have tests because the bits required to get integration-cli to play well are not there yet. So, please do not merge, but review carefully. Thanks!
Let’s discuss this one in IRC.

On Sep 5, 2014, at 1:46 AM, Alexandr Morozov <notifications@github.com> wrote:

> In pkg/execin/execin.go:
> 
> > @@ -0,0 +1,19 @@
> > +package execin
> > +
> > +import (
> > +	"os"
> > +	"runtime"
> > +
> > +	"github.com/docker/libcontainer/system"
> > +)
> > +
> > +func ExecIn(execFile *os.File, cloneOpts uintptr, enterFunc func() error) error {
> I think this is not very usable without reexec things, so possibly should be merged to links or new package as I proposed earlier.
> 
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