- Get _environ through the NSEnviron call in a MacOSX framework. This allows

us to completely decouple the framework from the executable, so we
  can use a two-level namespace.
- Do framework builds with a twolevel namespace.
- Reorganized the code that creates the minimal framework in the build
  directory, to make it more robust against incomplete frameworks (from
  earlier aborted builds, or builds of previous Python versions).