merge of gary-2-bootstrap-changes into trunk, resolving lots of conflicts with the distribute changes.

Need to check if tests are complete/sufficient, and test with multiple Pythons
and on Windows.

Changed zc.recipe.egg and the easy_install.scripts function into only adding
-S to scripts.

New package z3c.recipe.scripts and the easy_install.generate_scripts now
contains the more robust interpreter generation and the ability to
include site-packages.

Pertinent tests from the previous approach (from 3+ months ago) are
ported.  In the course of this port I discovered that big changes like
the ones in this checkin were needed.

an interpreter variant that supports all interpreter flags (AFAIK).  this commit contains core implementation (in easy_install module) and tests of all functionality except import_sitecustomize.  Integration into zc.recipe.egg and tests for import_sitecustomize (which will be more involved, and in tests.py) will follow.

this exposes the ability to request that site be imported in the ``script`` function. A later commit will expose this in the buildout configuration choices.

adjusted testselectingpython.py the same way as easyinstall.txt earlier: there's no call to _get_dist() for setuptools anymore as we already have it

Adjusted the test so that it also works when you're testing with an older setuptools version and there's a new one on pypi.
In that case, the _call_easy_install call will really try to install setuptools first and execute other code paths than the test cares for.
A cleaner way does not seem possible as the easy_install code is really intertwined.

Merged -r105475:105584 svn+ssh://reinout@svn.zope.org/repos/main/zc.buildout/branches/reinout_distribute_upgrade_check
This is the fix for an infinite recursion when updating Distribute.
Solved by using the available setuptools/distribute for easy_installing the Distribute tgz instead of searching for it, which tries to install, which searches...