Do not use up the ~WNOHANG waitpid call on completed processes

This is the more correct fix for #5447, as regardless of which process
in the job (be it the first or the last) finished first, once we have
waited on a process without ~WNOHANG we don't do that for any subsequent
processes in the job.

It is also a waste to call into the kernel to wait for a process we
already know is completed!

src/proc.cpp

@@ -478,6 +478,11 @@ static bool process_mark_finished_children(bool block_on_fg) {
                     break;
                 }
                 assert((*process)->pid != INVALID_PID && "Waiting by process on an invalid PID!");
+                if ((*process)->completed) {
+                    // This process has already been waited on to completion
+                    continue;
+                }
+
                 if ((options & WNOHANG) == 0) {
                     debug(4, "Waiting on individual process %d", (*process)->pid);
                 } else {