Work around broken Windows command-line processing.

It's wonderful that the Python interpreter will accept a whole multi-line
script as a composite -c argument... but because Windows command-line
processing is fundamentally flawed, we simply can't count on it for Windows.
Instead, accept script text, write a temporary script file in a system-
dependent temp directory, ask Python to run that script and delete the file.
Also, on Windows, use _spawnl(), much simpler than adding bizarre Windows wait
logic to LLProcessLauncher. Use LLProcessLauncher only on Mac & Linux, with
waitpid() to capture rc.