selftests: txtimestamp: add support for epoll().

Add the following new flags:
-e: use level-triggered epoll() instead of poll().
-E: use event-triggered epoll() instead of poll().

Signed-off-by: Jian Yang <jianyang@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>