Add os.waitstatus_to_exitcode() function to convert a wait status to an
exitcode.
Suggest waitstatus_to_exitcode() usage in the documentation when
appropriate.
Use waitstatus_to_exitcode() in:
* multiprocessing, os, subprocess and _bootsubprocess modules;
* test.support.wait_process();
* setup.py: run_command();
* and many tests.
* Add _waitstatus_to_exitcode() helper function to _bootsubprocess.
* Enhance check_output() error message if the command fails.
_bootsubprocess no longer handles WIFSTOPPED() case: it now raises a
ValueError.
Add _bootsubprocess module to bootstrap Python: subprocess
implementation which only uses the os module.
On AIX, distutils.util uses _aix_support which calls
subprocess.check_output(), before the _posixsubprocess module is
built. Implement check_output() with os.system() in _bootsubprocess.
