Shell: Correct program stop behaviour

Prior to this, we would only wait for program exit; the shell should
stop waiting once the program has been stopped.
Fixes #2230

Shell/main.cpp

@@ -1075,7 +1075,7 @@ static ExitCodeOrContinuationRequest run_command(const StringView& cmd)
         for (size_t i = 0; i < children.size(); ++i) {
             auto& child = children[i];
             do {
-                int rc = waitpid(child.pid, &wstatus, 0);
+                int rc = waitpid(child.pid, &wstatus, WSTOPPED);
                 if (rc < 0 && errno != EINTR) {
                     if (errno != ECHILD)
                         perror("waitpid");