Patch #683592 revisited, after discussions with MvL:

Patch #683592 revisited, after discussions with MvL:
- Implement the behavior as specified in PEP 277, meaning os.listdir()
  will only return unicode strings if it is _called_ with a unicode
  argument.
- And then return only unicode, don't attempt to convert to ASCII.
- Don't switch on Py_FileSystemDefaultEncoding, but simply use the
  default encoding if Py_FileSystemDefaultEncoding is NULL. This means
  os.listdir() can now raise UnicodeDecodeError if the default encoding
  can't represent the directory entry. (This seems better than silcencing
  the error and fall back to a byte string.)
- Attempted to decribe the above in Doc/lib/libos.tex.
- Reworded the Misc/NEWS items to reflect the current situation.

This checkin also fixes bug #696261, which was due to os.listdir() not
using Py_FileSystemDefaultEncoding, like all file system calls are
supposed to.