waitpid: implement WNOHANG for CONFIG_SCHED_HAVE_PARENT=y

If there's no child status available immediately,
return 0 without blocking as specified by the standards.

The implementation for non CONFIG_SCHED_HAVE_PARENT case
seems ok in this regard.

I checked the following version of the standard.
I believe it has always been this way though.

    The Open Group Base Specifications Issue 7, 2018 edition
    IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)

sched/sched/sched_waitpid.c

@@ -390,6 +390,12 @@ pid_t nx_waitpid(pid_t pid, int *stat_loc, int options)
 
 #endif /* CONFIG_SCHED_CHILD_STATUS */
 
+      if ((options & WNOHANG) != 0)
+        {
+          pid = 0;
+          break;
+        }
+
       /* Wait for any death-of-child signal */
 
       ret = nxsig_waitinfo(&set, &info);