This patch threads nested_scopes flags throughout the
interpreter (mostly pythonrun.c) to support execfile,
exec, and compile as described in PEP 236.

It also supports -i in the natural way.  If you run
python -i foo.py and foo.py contains a future
statement, that future statement is still in effect
when you reach the interactive interpreter prompt.

There are a mess of new functions that now take a
PyCompilerFlags * argument.  It's a total mess, but we
hopefully we can remove it all as soon as we release
2.2.  It's only necessary for 2.1 to support future
nested scopes.

Note that the Windows main.c may also need to be
modified.  I've got no idea if it has something like
-i.