Implement #1578269. Patch by Jason R. Coombs.

Added Windows support for os.symlink when run on Windows 6.0 or greater,
aka Vista. Previous Windows versions will raise NotImplementedError
when trying to symlink.

Includes numerous test updates and additions to test_os, including
a symlink_support module because of the fact that privilege escalation
is required in order to run the tests to ensure that the user is able
to create symlinks. By default, accounts do not have the required
privilege, so the escalation code will have to be exposed later (or
documented on how to do so). I'll be following up with that work next.

Note that the tests use ctypes, which was agreed on during the PyCon
language summit.

Lib/test/test_os.py

@@ -520,7 +520,7 @@ class WalkTests(unittest.TestCase):
             f = open(path, "w")
             f.write("I'm " + path + " and proud of it.  Blame test_os.\n")
             f.close()
-        if hasattr(os, "symlink"):
+        if support.can_symlink():
             os.symlink(os.path.abspath(t2_path), link_path)
             sub2_tree = (sub2_path, ["link"], ["tmp3"])
         else:
@@ -564,7 +564,7 @@ class WalkTests(unittest.TestCase):
         self.assertEqual(all[flipped + 1], (sub1_path, ["SUB11"], ["tmp2"]))
         self.assertEqual(all[2 - 2 * flipped], sub2_tree)
 
-        if hasattr(os, "symlink"):
+        if support.can_symlink():
             # Walk, following symlinks.
             for root, dirs, files in os.walk(walk_path, followlinks=True):
                 if root == link_path:
@@ -1033,6 +1033,83 @@ class Win32KillTests(unittest.TestCase):
         self._kill_with_event(signal.CTRL_BREAK_EVENT, "CTRL_BREAK_EVENT")
 
 
+def skipUnlessWindows6(test):
+    if hasattr(sys, 'getwindowsversion') and sys.getwindowsversion().major >= 6:
+        return test
+    return unittest.skip("Requires Windows Vista or later")(test)
+
+@unittest.skipUnless(sys.platform == "win32", "Win32 specific tests")
+@support.skip_unless_symlink
+class Win32SymlinkTests(unittest.TestCase):
+    filelink = 'filelinktest'
+    filelink_target = os.path.abspath(__file__)
+    dirlink = 'dirlinktest'
+    dirlink_target = os.path.dirname(filelink_target)
+    missing_link = 'missing link'
+
+    def setUp(self):
+        assert os.path.exists(self.dirlink_target)
+        assert os.path.exists(self.filelink_target)
+        assert not os.path.exists(self.dirlink)
+        assert not os.path.exists(self.filelink)
+        assert not os.path.exists(self.missing_link)
+
+    def tearDown(self):
+        if os.path.exists(self.filelink):
+            os.remove(self.filelink)
+        if os.path.exists(self.dirlink):
+            os.rmdir(self.dirlink)
+        if os.path.lexists(self.missing_link):
+            os.remove(self.missing_link)
+
+    def test_directory_link(self):
+        os.symlink(self.dirlink_target, self.dirlink)
+        self.assertTrue(os.path.exists(self.dirlink))
+        self.assertTrue(os.path.isdir(self.dirlink))
+        self.assertTrue(os.path.islink(self.dirlink))
+        self.check_stat(self.dirlink, self.dirlink_target)
+
+    def test_file_link(self):
+        os.symlink(self.filelink_target, self.filelink)
+        self.assertTrue(os.path.exists(self.filelink))
+        self.assertTrue(os.path.isfile(self.filelink))
+        self.assertTrue(os.path.islink(self.filelink))
+        self.check_stat(self.filelink, self.filelink_target)
+
+    def _create_missing_dir_link(self):
+        'Create a "directory" link to a non-existent target'
+        linkname = self.missing_link
+        if os.path.lexists(linkname):
+            os.remove(linkname)
+        target = r'c:\\target does not exist.29r3c740'
+        assert not os.path.exists(target)
+        target_is_dir = True
+        os.symlink(target, linkname, target_is_dir)
+
+    def test_remove_directory_link_to_missing_target(self):
+        self._create_missing_dir_link()
+        # For compatibility with Unix, os.remove will check the
+        #  directory status and call RemoveDirectory if the symlink
+        #  was created with target_is_dir==True.
+        os.remove(self.missing_link)
+
+    @unittest.skip("currently fails; consider for improvement")
+    def test_isdir_on_directory_link_to_missing_target(self):
+        self._create_missing_dir_link()
+        # consider having isdir return true for directory links
+        self.assertTrue(os.path.isdir(self.missing_link))
+
+    @unittest.skip("currently fails; consider for improvement")
+    def test_rmdir_on_directory_link_to_missing_target(self):
+        self._create_missing_dir_link()
+        # consider allowing rmdir to remove directory links
+        os.rmdir(self.missing_link)
+
+    def check_stat(self, link, target):
+        self.assertEqual(os.stat(link), os.stat(target))
+        self.assertNotEqual(os.lstat(link), os.stat(link))
+
+
 class MiscTests(unittest.TestCase):
 
     @unittest.skipIf(os.name == "nt", "POSIX specific test")
@@ -1056,6 +1133,7 @@ def test_main():
         PosixUidGidTests,
         Pep383Tests,
         Win32KillTests,
+        Win32SymlinkTests,
         MiscTests,
     )