initgroups(2) is a function for initializing the aux group id list from
site-specific configuration (typically /etc/groups, but not necessarily).

Attached is a patch based on some code from Zope, expanded to have a
test and some docs.  Presumably Zope will be happy to use this function
from the stdlib once they can depend on a version of Python which
includes it.  Twisted would like to as well.

I wasn't able to test the autoconf related parts of the patch as I don't
have easy access to autoconf 2.61.

I also can't run regrtest.py in my environment for some reason.  It
fails to import some names from test_support.  So I didn't run the unit
test that I added, either.

In case anyone is concerned about licensing issues, please see
<https://mail.zope.org/pipermail/zope-dev/2009-November/038289.html>

The test should skip when not hasattr(os, 'initgroups') since it's 
optional

Good point.  Updated patch attached.

You can use the new test-skipping feature:
http://docs.python.org/dev/library/unittest.html#skipping-tests-and-expected-failures

Alright.  Updated patch attached.

pyflakes told me I didn't import errno in test_posix.py.  Updated patch
attached.

The documentation is os.rst is too laconic (coming from the Windows
world, I could not understand what the function is about).
The docstring is much better (at least I understand it's related to the
Unix security model), I suggest to use it for the documentation.

New version attached which makes the os.rst docs the same as the
docstring.  Also fixes a conflict since introduced in trunk in configure.in.

Slightly modified and committed in r76636 (trunk) and r76637 (py3k).
Thank you.