gh-116522: Refactor _PyThreadState_DeleteExcept (#117131)

Split `_PyThreadState_DeleteExcept` into two functions:

- `_PyThreadState_RemoveExcept` removes all thread states other than one
  passed as an argument. It returns the removed thread states as a
  linked list.

- `_PyThreadState_DeleteList` deletes those dead thread states. It may
  call destructors, so we want to "start the world" before calling
  `_PyThreadState_DeleteList` to avoid potential deadlocks.

Modules/posixmodule.c

@@ -664,6 +664,14 @@ PyOS_AfterFork_Child(void)
         goto fatal_error;
     }
 
+    // Remove the dead thread states. We "start the world" once we are the only
+    // thread state left to undo the stop the world call in `PyOS_BeforeFork`.
+    // That needs to happen before `_PyThreadState_DeleteList`, because that
+    // may call destructors.
+    PyThreadState *list = _PyThreadState_RemoveExcept(tstate);
+    _PyEval_StartTheWorldAll(&_PyRuntime);
+    _PyThreadState_DeleteList(list);
+
     status = _PyImport_ReInitLock(tstate->interp);
     if (_PyStatus_EXCEPTION(status)) {
         goto fatal_error;