Backport PR #2799: Exit code

Correct the exit code to be a simple value, rather than two or three separate values stuffed into a single integer.

Closes issue  #2794.

My reading of the [Windows docs](http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/exit.mspx?mfr=true) is that the test I've added should work on Windows as well.

IPython/core/interactiveshell.py

@@ -2206,6 +2206,10 b' class InteractiveShell(SingletonConfigurable):'
         else:
             cmd = py3compat.unicode_to_str(cmd)
             ec = os.system(cmd)
+            # The high byte is the exit code, the low byte is a signal number
+            # that we discard for now. See the docs for os.wait()
+            if ec > 255:
+                ec >>= 8
         
         # We explicitly do NOT return the subprocess status code, because
         # a non-None value would trigger :func:`sys.displayhook` calls.

IPython/core/tests/test_interactiveshell.py

@@ -402,6 +402,11 b' class TestSystemRaw(unittest.TestCase):'
         cmd = ur'''python -c "'åäö'"   '''
         ip.system_raw(cmd)
 
+    def test_exit_code(self):
+        """Test that the exit code is parsed correctly."""
+        ip.system_raw('exit 1')
+        self.assertEqual(ip.user_ns['_exit_code'], 1)
+
 class TestModules(unittest.TestCase, tt.TempFileMixin):
     def test_extraneous_loads(self):
         """Test we're not loading modules on startup that we shouldn't.