[MLton] a few remaining Solaris bugs with the new runtime

[Matthew Fluet]

> I've successfully cross-compiled MLton to run on Solaris with the new
> runtime.  The resulting MLton compiles all the regressions, and there
> is only one regression failure: echo.sml, which reports
>
>  unhandled exception: Error
>
> instead of
>
>  server processed 1900 bytes

I remember seeing this with x86-linux, but I fixed it somehow.
I'm not entirely convinced that our network/host conversions are all 
correct.  I'd suggest starting with the other bugs.