[MLton] interrupted system call

[Matthew Fluet]

> > I'd really like to add
> >       structure AtomicState :
> > 	 sig
> > 	    datatype t = NonAtomic | Atomic of int
> > 	 end
> >       val atomicState: unit -> AtomicState.t
> > to MLton.Thread.  It's useful for asserts in ML.
>
> OK.  I see that signals2 regression uses AtomicState, which isn't
> checked in yet.

I removed that part of the code; it's irrelevant to the regression.

> Is NonAtomic the same as canHandle = 0?

Yes.