[core] Decode environment variables with filesystem encoding (Fixes #3854, Fixes #3217, Fixes #2918)

Introduces compat versions of os.getenv and os.path.expanduser

youtube_dl/cache.py

@@ -9,6 +9,7 @@ import shutil
 import traceback
 
 from .utils import (
+    compat_expanduser,
     write_json_file,
 )
 
@@ -22,7 +23,7 @@ class Cache(object):
         if res is None:
             cache_root = os.environ.get('XDG_CACHE_HOME', '~/.cache')
             res = os.path.join(cache_root, 'youtube-dl')
-        return os.path.expanduser(res)
+        return compat_expanduser(res)
 
     def _get_cache_fn(self, section, key, dtype):
         assert re.match(r'^[a-zA-Z0-9_.-]+$', section), \