Kernel + LibC: Handle running processes in do_waitid()

Author: https://github.com/alimpfard
Commit: 8582a06899
Pull-request: https://github.com/SerenityOS/serenity/pull/2241
Reviewed-by: https://github.com/Dexesttp
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/bugaevc

Kernel/Scheduler.cpp

@@ -274,7 +274,8 @@ bool Thread::WaitBlocker::should_unblock(Thread& thread, time_t, long)
         }
 
         bool wait_finished = ((m_wait_options & WEXITED) && child_exited)
-            || ((m_wait_options & WSTOPPED) && child_stopped);
+            || ((m_wait_options & WSTOPPED) && child_stopped)
+            || (m_wait_options & WNOHANG);
 
         if (!wait_finished)
             return IterationDecision::Continue;