sched/sched/sched_waitid.c: Discard the child entry

Summary:
 - Discard the child entry after receiving SIGCHLD.

Impact:
 - waitid

Testing:
hifive1-revb:nsh (CONFIG_SCHED_HAVE_PARENT=y, CONFIG_SCHED_CHILD_STATUS=y, CONFIG_SIG_DEFAULT=y)
on QEMU

static int task_main(int argc, char *argv[])
{
  sleep(1);
  return 0;
}

int main(int argc, FAR char *argv[])
{
  pid_t pid;
  siginfo_t info;
  int ret;

  pid = task_create("task1", 224, 1024, task_main, NULL);
  task_create("task2", 224, 1024, task_main, NULL);

  waitid(P_PID, pid, &info, WEXITED);
  waitid(P_ALL, 0, &info, WEXITED);
  ret = waitid(P_ALL, 0, &info, WEXITED | WNOHANG);

  assert(ret == -1);

  return 0;
}

sched/sched/sched_waitid.c

@@ -74,12 +74,15 @@ static void exited_child(FAR struct tcb_s *rtcb,
    * information).
    */
 
+  if (info)
+    {
       info->si_signo           = SIGCHLD;
       info->si_code            = CLD_EXITED;
       info->si_errno           = OK;
       info->si_value.sival_ptr = NULL;
       info->si_pid             = child->ch_pid;
       info->si_status          = child->ch_status;
+    }
 
   /* Discard the child entry */
 
@@ -364,6 +367,19 @@ int nx_waitid(int idtype, id_t id, FAR siginfo_t *info, int options)
                 {
                   /* Yes... return success */
 
+#ifdef CONFIG_SCHED_CHILD_STATUS
+                  if (retains)
+                    {
+                      child = group_find_child(rtcb->group, info->si_pid);
+                      DEBUGASSERT(child);
+
+                      if ((child->ch_flags & CHILD_FLAG_EXITED) != 0)
+                        {
+                          exited_child(rtcb, child, NULL);
+                        }
+                    }
+#endif
+
                   break;
                 }
             }
@@ -374,6 +390,19 @@ int nx_waitid(int idtype, id_t id, FAR siginfo_t *info, int options)
             {
               /* Return success */
 
+#ifdef CONFIG_SCHED_CHILD_STATUS
+                  if (retains)
+                    {
+                      child = group_find_child(rtcb->group, info->si_pid);
+
+                      if (child &&
+                          (child->ch_flags & CHILD_FLAG_EXITED) != 0)
+                        {
+                          exited_child(rtcb, child, NULL);
+                        }
+                    }
+#endif
+
               break;
             }