Add a pkg for iptables support

Depends on #1592 (exec support, duped into this PR).
Multiple goroutines is at least under our control.  If it's not worth handling in proc races I'd add a comment to the package doc to that effect.


> On Oct 6, 2014, at 7:58 PM, Tim Hockin <notifications@github.com> wrote:
> 
> In pkg/util/iptables/iptables.go:
> 
> > +
> > +// FlushChain is part of Interface.
> > +func (runner *runner) FlushChain(table Table, chain Chain) error {
> > +	fullArgs := makeFullArgs(table, chain)
> > +
> > +	out, err := runner.run(opFlushChain, fullArgs)
> > +	if err != nil {
> > +		glog.Errorf("Error flushing chain %q: %s: %s", chain, err, out)
> > +		return err
> > +	}
> > +	return nil
> > +}
> > +
> > +// EnsureRule is part of Interface.
> > +func (runner *runner) EnsureRule(table Table, chain Chain, args ...string) (bool, error) {
> > +	fullArgs := makeFullArgs(table, chain, args...)
> Specifically that multiple programs can be iptabling at the same time on the machine? I don't see how this can solve that problem.
> 
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