[MLton] overflow vs. SysErr on time functions

[Matthew Fluet]

> As I alluded to on the Basis Library discussion list, I'm planning to
> change the following functions so that they raise SysErr inval, not
> overflow on time values that are too large.
>
> 	OS.FileSys.setTime
> 	OS.IO.poll
> 	OS.Process.sleep
> 	Posix.FileSys.utime
> 	Posix.Process.{alarm,sleep}
> 	Socket.select
>
> Any objections/support?

I don't object, but I think there is a small hole that no Basis Library
function fills.  In particular, there is no way to check the validity of a
time value for these system calls.