gh-90473: Skip and document more failing tests on WASI (GH-93436)

- Mark more ``umask()`` cases
- ``dup()`` is not supported
- ``/dev/null`` is not available
- document missing features
- mark more modules as not available
(cherry picked from commit 069c96f84c)

Co-authored-by: Christian Heimes <christian@python.org>

Lib/test/test_shutil.py

@@ -2652,6 +2652,7 @@ class TestGetTerminalSize(unittest.TestCase):
 
         self.assertEqual(expected, actual)
 
+    @unittest.skipIf(support.is_wasi, "WASI has no /dev/null")
     def test_fallback(self):
         with os_helper.EnvironmentVarGuard() as env:
             del env['LINES']