- Move a test from call time to define time
- Add the function name to __all__
- Improve docstring and docs

A few lines are now duplicated (named tuple definition and docstring)
but I think the end result reads better.

In the install and library docs, I changed the text to refer to
packaging instead of distutils.  I also checked that the documented
paths correctly reflect what’s really defined in sysconfig; the main
difference with paths defined in distutils.install is that include
directories don’t end with the distribution name anymore (i.e. distutils
uses include/python3.3/spam, sysconfig include/python3.3), I have no
idea why.

This started out as an easy task, just add a section describing this
alternate scheme, but I found a lot of cleanup to do along the way:

- fixed inverted reST targets
- fixed entries for modules (hi abiflags!) or data files
- avoided duplicating the same options listing five or six times
- added missing entries for C headers locations
- added documentation for --install-lib
- fixed a few misuses of the option role (see #9312), but not all (not
  worth the time, but will do it in packaging docs)
- fixed some markup

The paths fixes were done with an eye on the source code in the install
command, so they really describe what’s actually done.  The situation on
Mac OS X is rather messy: the fix for #8084 touched site and sysconfig,
but distutils doesn’t use these files.  I suspect we have a mismatched
stdlib at the moment, and the fix is not even clear (see the bug report
for further discussion).

site.USER_BASE and site.USER_SITE are now fully documented.  PEP 370 is
outdated with respects to the Mac framework situation, but the code in
sysconfig and the example in the 3.2 What’s New document helped me find
the right values to document for Mac OS X.

The command-line interface of the site module, partly documented in the
3.2 What’s New, is fully described in the module docs.

The purpose of the usercustomize module is explained in the site docs,
with a gentle introduction in the tutorial (right after the section that
talks about PYTHONSTARTUP; a comment mentions it should be moved from
the tutorial to another file, but that will be another bug).

Various markup and wording improvements were made along the way in the
site module docs.  Duplicate and incomplete declarations of environment
variables have also been removed (the original bug report was actually
about these entries :).  The site module docs are still a bit messy;
I’ll see about improving them for #11553.

All these sections are copiously interlinked and findable from the doc
indexes.

in os.kill().  Original patch by Eli Bendersky.

in os.kill().  Original patch by Eli Bendersky.