gh-115035: Mark ThreadHandles as non-joinable earlier after forking (#115042)

This marks dead ThreadHandles as non-joinable earlier in
`PyOS_AfterFork_Child()` before we execute any Python code. The handles
are stored in a global linked list in `_PyRuntimeState` because `fork()`
affects the entire process.

Python/pystate.c

@@ -517,6 +517,8 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
         return _PyStatus_NO_MEMORY();
     }
 
+    _PyThread_AfterFork(&runtime->threads);
+
     return _PyStatus_OK();
 }
 #endif

Python/thread_nt.h

@@ -242,10 +242,6 @@ PyThread_detach_thread(PyThread_handle_t handle) {
     return (CloseHandle(hThread) == 0);
 }
 
-void
-PyThread_update_thread_after_fork(PyThread_ident_t* ident, PyThread_handle_t* handle) {
-}
-
 /*
  * Return the thread Id instead of a handle. The Id is said to uniquely identify the
  * thread in the system

Python/thread_pthread.h

@@ -339,16 +339,6 @@ PyThread_detach_thread(PyThread_handle_t th) {
     return pthread_detach((pthread_t) th);
 }
 
-void
-PyThread_update_thread_after_fork(PyThread_ident_t* ident, PyThread_handle_t* handle) {
-    // The thread id might have been updated in the forked child
-    pthread_t th = pthread_self();
-    *ident = (PyThread_ident_t) th;
-    *handle = (PyThread_handle_t) th;
-    assert(th == (pthread_t) *ident);
-    assert(th == (pthread_t) *handle);
-}
-
 /* XXX This implementation is considered (to quote Tim Peters) "inherently
    hosed" because:
      - It does not guarantee the promise that a non-zero integer is returned.