[util] Move compatibility functions out of util

utils is large enough without these compatibility functions.

Everything that is present in newer versions of Python (i.e. with dev Python it's just an import) goes into compat.py .
Everything else (i.e. youtube-dl-specific helpers) goes into utils.py .

test/test_compat.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python
+# coding: utf-8
+
+from __future__ import unicode_literals
+
+# Allow direct execution
+import os
+import sys
+import unittest
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+
+from youtube_dl.utils import get_filesystem_encoding
+from youtube_dl.compat import (
+    compat_getenv,
+    compat_expanduser,
+)
+
+
+class TestCompat(unittest.TestCase):
+    def test_compat_getenv(self):
+        test_str = 'тест'
+        os.environ['YOUTUBE-DL-TEST'] = (
+            test_str if sys.version_info >= (3, 0)
+            else test_str.encode(get_filesystem_encoding()))
+        self.assertEqual(compat_getenv('YOUTUBE-DL-TEST'), test_str)
+
+    def test_compat_expanduser(self):
+        test_str = 'C:\Documents and Settings\тест\Application Data'
+        os.environ['HOME'] = (
+            test_str if sys.version_info >= (3, 0)
+            else test_str.encode(get_filesystem_encoding()))
+        self.assertEqual(compat_expanduser('~'), test_str)
+
+    def test_all_present(self):
+        import youtube_dl.compat
+        all_names = youtube_dl.compat.__all__
+        present_names = set(filter(
+            lambda c: '_' in c and not c.startswith('_'),
+            dir(youtube_dl.compat)))
+        self.assertEqual(all_names, sorted(present_names))
+
+if __name__ == '__main__':
+    unittest.main()