Add an example to use epoll-based rxdump.

- Run "make EPOLL=1" to make it use epoll.
- Currently this exhibits about 3% performance degradation compared to
  the original rxdump that used blocking ps_recv_chunk() calls.
  This might be due to duplicate linear search over multiple queues on
  epoll_wait() and ps_recv_chunk().  We need to inspect this effect on
  various multi-threaded & batching conditions.