Moved clear_carefully() to _PyModule_Clear() in moduleobject.c

Moved clear_carefully() to _PyModule_Clear() in moduleobject.c
(modified) and use that.

Some differences in the cleanup algorithm:

- Clear __main__ before the other modules.

- Delete more sys variables: including ps1, ps2, exitfunc, argv, and
even path -- this will prevent new imports!

- Restore stdin, stdout, stderr from __stdin__, __stdout__,
__stderr__, effectively deleting hooks that the user might have
installed -- so their (the hooks') destructors will run.