Implement the polling interface for the RX part.

 - Now the user-level apps can use select/poll/epoll to check if any
   packets has arrived to the attached queues for their handles.
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.
Minor optimization for subsequent calls of epoll_wait() and ps_recv_chunk().

- No visible performance improvements, though.
Implement random-sized packet generation which verifies #8.

 - Use `--min-pkt-size` option after `-p` option to activate.