Router: introducing the PORT_ACK message.

The PORT_ACK message is the router's response to the application's NEW_PORT
message.  After receiving PORT_ACK, the application is safe to process requests
using this port.

This message avoids a racing condition when the application starts processing a
request from the shared queue and sends REQ_HEADERS_ACK.  The REQ_HEADERS_ACK
message contains the application port ID as reply_port, which the router uses
to send request data.  When the application creates a new port, it
immediately sends it to the main router thread.  Because the request is
processed outside the main thread, a racing condition can occur between the
receipt of the new port in the main thread and the receipt of REQ_HEADERS_ACK
in the worker router thread where the same port is specified as reply_port.

src/nxt_unit.h

@@ -154,6 +154,8 @@ struct nxt_unit_callbacks_s {
     /* Receive data on port id. Optional. */
     ssize_t  (*port_recv)(nxt_unit_ctx_t *, nxt_unit_port_t *port,
                  void *buf, size_t buf_size, void *oob, size_t oob_size);
+
+    int      (*ready_handler)(nxt_unit_ctx_t *);
 };