Shadow's epoll and poll don't support EPOLLERR and EPOLLHUP

[stevenengler]

Events for poll and epoll include EPOLLHUP/POLLHUP and EPOLLERR/POLLERR to indicate hang up / error conditions on files. Shadow currently only supports EPOLLIN and EPOLLOUT for level-triggered events, so it does not correctly indicate these hang up / error conditions.

We also need better tests for these conditions, and for the behaviour of select(). I think sometimes a file where epoll would return EPOLLHUP and not EPOLLIN will still appear in the readfds list for select().