Support os.walk(dir_fd=) and os.fwalk(dir_fd=)

Doc/library/os.rst

 :mod:`os` --- Miscellaneous operating system interfaces
 =======================================================
 
 .. module:: os
    :synopsis: Miscellaneous operating system interfaces.
 
 
 This module provides a portable way of using operating system dependent
 functionality.  If you just want to read or write a file see :func:`open`, if
 you want to manipulate paths, see the :mod:`os.path` module, and if you want to
@@ skipping 2190 matching lines @@
       # CAUTION:  This is dangerous!  For example, if top == '/', it
       # could delete all your disk files.
       import os
       for root, dirs, files in os.walk(top, topdown=False):
           for name in files:
               os.remove(os.path.join(root, name))
           for name in dirs:
               os.rmdir(os.path.join(root, name))
 
 
-.. function:: fwalk(top, topdown=True, onerror=None, followlinks=False)
+.. function:: fwalk(top='.', topdown=True, onerror=None, followlinks=False, *, dir_fd=None)
 
    .. index::
       single: directory; walking
       single: directory; traversal
 
    This behaves exactly like :func:`walk`, except that it yields a 4-tuple
-   ``(dirpath, dirnames, filenames, dirfd)``.
+   ``(dirpath, dirnames, filenames, dirfd)``, and it supports ``dir_fd``.

[storchaka, 2012/06/25 14:08:10]

But walk also supports dir_fd.

[larry, 2012/06/25 15:04:13]

Wishful thinking for now ;-) but, Done.

 
    *dirpath*, *dirnames* and *filenames* are identical to :func:`walk` output,
    and *dirfd* is a file descriptor referring to the directory *dirpath*.
 
+   This function always supports :ref:`paths relative to directory descriptors
+   <dir_fd>`.
+
    .. note::
 
       Since :func:`fwalk` yields file descriptors, those are only valid until
       the next iteration step, so you should duplicate them (e.g. with
       :func:`dup`) if you want to keep them longer.
 
    This example displays the number of bytes taken by non-directory files in each
    directory under the starting directory, except that it doesn't look under any
    CVS subdirectory::
 
@@ skipping 1085 matching lines @@
 
 .. function:: urandom(n)
 
    Return a string of *n* random bytes suitable for cryptographic use.
 
    This function returns random bytes from an OS-specific randomness source.  The
    returned data should be unpredictable enough for cryptographic applications,
    though its exact quality depends on the OS implementation.  On a Unix-like
    system this will query /dev/urandom, and on Windows it will use CryptGenRandom.
    If a randomness source is not found, :exc:`NotImplementedError` will be raised.