task/task_spawnparms.c: Set the new process's signal mask

Set the newly spawned process's signal mask, if the caller has instructed
to do so by setting POSIX_SPAWN_SETSIGMASK.

This is called after the task has been created but has NOT been started
yet.

sched/task/task_spawnparms.c

@@ -149,6 +149,17 @@ int spawn_execattrs(pid_t pid, FAR const posix_spawnattr_t *attr)
    * return an error value, then we would also have to stop the task.
    */
 
+  /* Firstly, set the signal mask if requested to do so */
+
+  if ((attr->flags & POSIX_SPAWN_SETSIGMASK) != 0)
+    {
+      FAR struct tcb_s *tcb = nxsched_get_tcb(pid);
+      if (tcb)
+        {
+          tcb->sigprocmask = attr->sigmask;
+        }
+    }
+
   /* If we are only setting the priority, then call sched_setparm()
    * to set the priority of the of the new task.
    */