Rewritten PyImport_Cleanup() and its helper, clear_carefully(). They

now implement the following finalization strategy.

1. Whenever this code deletes a module, its directory is cleared
   carefully, as follows:
   - set all names to None that begin with exactly one underscore
   - set all names to None that don't begin with two underscores
   - clear the directory

2. Modules are deleted in the following order:
   - modules with a reference count of 1, except __builtin__ or __sys__
   - repeat until no more are found with a reference count of 1
   - __main__ if it's still there
   - all remaining modules except __builtin__ or sys
   - sys
   _ __builtin__