fixes for bootstrap and a system Python; changes based on learning what would be necessary to be able to develop buildout with a system Python (zc.recipe.testing would also need to use sitepackage_safe_scripts)

with these changes, I can build zc.buildout and run its tests successfully with my system Python.  To make it fully robust, zc.recipe.test probably would need to use sitepackage_safe_scripts, but this works for now.

settle on "include-site-packages" and migrate all variations of "add-site-packages" to that spelling.

integrate ability to control what eggs are accepted from site-packages into z3c.recipe.scripts.  Without this, the ability to use z3c.recipe.scripts without site-packages was broken.

this is now the branch for my site-packages changes.  It is merged with trunk.  The zc.recipe.egg recipe returns to being fully backward compatible; if you want to use a system Python, use the z3c.recipe.scripts recipe for scripts and interpreter.