Two more refinements of the cleanup process.

Two more refinements of the cleanup process.

(1) Explicitly clear __builtin__._ and sys.{last,exc}_* before
clearing anything else.  These are common places where user values
hide and people complain when their destructors fail.  Since the
modules containing them are deleted *last* of all, they would come too
late in the normal destruction order.  Sigh.

(2) Add some debugging aid to cleanup (after a suggestion by Marc
Lemburg) -- print the names of the modules being cleaned, and (when
-vv is used) print the names of the variables being cleared.