Fixed deferred accept with EPOLLRDHUP enabled (ticket #1278).

Fixed deferred accept with EPOLLRDHUP enabled (ticket #1278).

Previously, the read event of the accepted connection was marked ready, but not
available.  This made EPOLLRDHUP-related code (for example, in ngx_unix_recv())
expect more data from the socket, leading to unexpected behavior.

For example, if SSL, PROXY protocol and deferred accept were enabled on a listen
socket, the client connection was aborted due to unexpected return value of
c->recv().