Listen for only EPOLLIN events with listeners

The example in the epoll(7) manpage only sets the EPOLLIN event flag.

Although it is not explicitly stated that only EPOLLIN events arrive for
sockets that are listening, any other types of events should not be
relevant for listeners.

server/core/dcb.cc

@@ -3451,6 +3451,12 @@ int poll_add_dcb(DCB *dcb)
     }
     else
     {
+        /**
+         * Listeners are always added in level triggered mode. This will cause
+         * new events to be generated as long as there is at least one connection
+         * to accept.
+         */
+        events = EPOLLIN;
         ss_dassert(dcb->dcb_role == DCB_ROLE_SERVICE_LISTENER);
         new_state = DCB_STATE_LISTENING;
     }