Hum, I changed my mind and I think that it's worth it to make the function public. Moreover, I prefer "exitcode", since it is closer to "WEXITSTATUS" name than "returncode". So I renamed the function os.status_to_exitcode().

Advantages of the new new function compared to exiting code:

* the function guarantee that result >= 0 means that the process exited and result < 0 means that the process exited due to a signal (was killed by a signal)
* the function raises a well defined exception (ValueError) if the WIFSTOPPED() is true or if the status is unknown: it prevents to misuse WIFSTOPPED()