[MLton] interrupted system call

[Stephen Weeks]

> 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.

Is NonAtomic the same as canHandle = 0?

> The inc/dec of canHandle is cheap.  We can drop the if
> (signalIsPending) part of the code in the absence of handlesSignals.

Sounds good.