MXS-2196: Remove DCB from Listener

Replaced the DCB with a single file descriptor that the listener listens
on and which is added to all of the workers. The Listener also extends the
MXB_POLL_DATA which allows it to handle epoll events.

Moved the code that creates the listening socket into listener.cc where it
belongs and did a minor cleanup of it.

server/core/config_runtime.cc

@@ -1022,7 +1022,7 @@ bool runtime_create_listener(Service* service,
                 MXS_NOTICE("Created %slistener '%s' at %s:%s for service '%s'",
                            ssl ? "TLS encrypted " : "", name, print_addr, port, service->name);
 
-                if (listener->listen(listener))
+                if (listener->listen())
                 {
                     rval = true;
                 }