[MLton] Win32 Spawn's Seem Broken

[Matthew Fluet]

> 1.  Modify spawnp/spawne to use "NON_BLOCK" so that it
> returns a PID, rather than a return code?

I suspect this will be the right solution; you may need to use appropriate
  #if (defined (__MINGW__))
magic to avoid conflicts with Cygwin.

I note that in  Cygwin's /usr/include/process.h, I have:

#define _P_WAIT         1
#define _P_NOWAIT       2       /* always generates error */
#define _P_OVERLAY      3
#define _P_NOWAITO      4
#define _P_DETACH       5

even though NOWAIT would seem to be the appropriate flag.  I don't know if
this is due to Cygwin or Win32.

I've never worked on the Cygwin port, so experimentation may be your best
bet.