[MLton] overflow vs. SysErr on time functions

[Stephen Weeks]

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?