Win32 fixes : process.cabal and test 3994 (win32 process must be started before it can be interrupted)

Avoids a name collision on Windows

 * The return type is not 'Either (ExitCode, String) String', it is just
   'String'
 * readProcess does not return stderr + stdout, it returns just stdout

`sleep 1s' is probably a GNUism; `sleep 1' should do the same in a
portable way.