bpo-40094: Add os.waitstatus_to_exitcode() (GH-19201)

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.

1 files changed, 3 insertions, 2 deletions

diff --git a/Lib/test/test_popen.py b/Lib/test/test_popen.py
index da01a87..ab1bc77 100644
--- a/Lib/test/test_popen.py
+++ b/Lib/test/test_popen.py
@@ -44,10 +44,11 @@ class PopenTest(unittest.TestCase):
 
     def test_return_code(self):
         self.assertEqual(os.popen("exit 0").close(), None)
+        status = os.popen("exit 42").close()
         if os.name == 'nt':
-            self.assertEqual(os.popen("exit 42").close(), 42)
+            self.assertEqual(status, 42)
         else:
-            self.assertEqual(os.popen("exit 42").close(), 42 << 8)
+            self.assertEqual(os.waitstatus_to_exitcode(status), 42)
 
     def test_contextmanager(self):
         with os.popen("echo hello") as f: