Update thread and syscall to use local fd counter instead global one, preventing overflow

src/threads/thread.c

@@ -688,6 +688,7 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->recent_cpu = recent_cpu;
   t->priority = t->base_priority;
 
+  t->fd_counter = MINIMUM_USER_FD;
   t->exit_status = -1;
   list_init (&t->child_results);