process.c: Call rb_thread_atfork in rb_fork_ruby

All occurrences of rb_fork_ruby are followed by a call rb_thread_fork in
the created child process.

This is refactoring and a potential preparation for [Feature #17795].
(rb_fork_ruby may be wrapped by Process._fork_.)

io.c

@@ -6871,7 +6871,6 @@ pipe_open(VALUE execarg_obj, const char *modestr, int fmode,
 # if defined(HAVE_WORKING_FORK)
 	pid = rb_fork_ruby(&status);
 	if (pid == 0) {		/* child */
-	    rb_thread_atfork();
 	    popen_redirect(&arg);
 	    rb_io_synchronized(RFILE(orig_stdout)->fptr);
 	    rb_io_synchronized(RFILE(orig_stderr)->fptr);

process.c

@@ -4293,8 +4293,10 @@ rb_fork_ruby(int *status)
         after_fork_ruby();
 	disable_child_handler_fork_parent(&old); /* yes, bad name */
         if (mjit_enabled && pid > 0) mjit_resume(); /* child (pid == 0) is cared by rb_thread_atfork */
-	if (pid >= 0) /* fork succeed */
+	if (pid >= 0) { /* fork succeed */
+            if (pid == 0) rb_thread_atfork();
 	    return pid;
+        }
 	/* fork failed */
 	if (handle_fork_error(err, status, NULL, &try_gc))
 	    return -1;
@@ -4336,7 +4338,6 @@ rb_f_fork(VALUE obj)
 
     switch (pid = rb_fork_ruby(NULL)) {
       case 0:
-	rb_thread_atfork();
 	if (rb_block_given_p()) {
 	    int status;
 	    rb_protect(rb_yield, Qundef, &status);
@@ -7012,7 +7013,7 @@ rb_daemon(int nochdir, int noclose)
 #define fork_daemon() \
     switch (rb_fork_ruby(NULL)) { \
       case -1: return -1; \
-      case 0:  rb_thread_atfork(); break; \
+      case 0:  break; \
       default: _exit(EXIT_SUCCESS); \
     }