gh-83856: Honor atexit for all multiprocessing start methods (GH-114279)

Use atexit for all multiprocessing start methods to cleanup.
See the GH-114279 PR discussion and related issue for details as to why.

Lib/multiprocessing/popen_fork.py

@@ -1,3 +1,4 @@
+import atexit
 import os
 import signal
 
@@ -66,10 +67,13 @@ def _launch(self, process_obj):
         self.pid = os.fork()
         if self.pid == 0:
             try:
+                atexit._clear()
+                atexit.register(util._exit_function)
                 os.close(parent_r)
                 os.close(parent_w)
                 code = process_obj._bootstrap(parent_sentinel=child_r)
             finally:
+                atexit._run_exitfuncs()
                 os._exit(code)
         else:
             os.close(child_w)