This does:

- naming cleanup: ep_* -> eventpoll_* for non-static functions ( 2 )

- No more limit of 2 poll wait queue for each file*

Before epoll used to have, inside its item struct, space for two wait
queues. This was driven by the fact that during a f_op->poll() each file
won't register more than one read and one write wait queue. Now, I'm not
sure if this is 100% true or not, but with the current implementation a
linked list of wait queues is kept to remove each limit.