poll: Prevent libevent from spinning if POLLNVAL occurs

poll: Prevent libevent from spinning if POLLNVAL occurs

This can happen, for example if libevent is being used to poll fds given
by another library where the other library closes the fds without
notifying the program using it that said fds were closed. In this case,
libevent will simply spin on poll() since there are active fds, but
won't call any event callback to handle the condition.

In epoll case after socket closed it automatically removed from epfd, so
IOW it will not spin in epoll* API, just a timeout.

Fixes: #379