Support legacy encodings better for synchronization

I have encountered another piece of code, which hangs the
sync process due to incompatibility with my legacy non-UTF8
locale. So I've added one more function to gpodder.util
which just cleans strings from possible further errors.

src/gpodder/sync.py

@@ -490,7 +490,7 @@ class MP3PlayerDevice(Device):
         else:
             folder = self.destination
 
-        from_file = episode.local_filename()
+        from_file = util.sanitize_encoding(episode.local_filename())
         filename_base = util.sanitize_filename(episode.sync_filename(), self.MAX_FILENAME_LENGTH)
 
         to_file = filename_base + os.path.splitext(from_file)[1].lower()

src/gpodder/util.py

@@ -934,6 +934,15 @@ def open_website(url):
     """
     threading.Thread(target=webbrowser.open, args=(url,)).start()
 
+def sanitize_encoding(filename):
+    """
+    Generate a sanitized version of a string (i.e.
+    remove invalid characters and encode in the
+    detected native language encoding)
+    """
+    global encoding
+    return filename.encode(encoding, 'ignore')
+
 
 def sanitize_filename(filename, max_length=0):
     """