Sending shared port to application prototype.

Application process started with shared port (and queue) already configured.
But still waits for PORT_ACK message from router to start request processing
(so-called "ready state").

Waiting for router confirmation is necessary.  Otherwise, the application may
produce response and send it to router before the router have the information
about the application process.  This is a subject of further optimizations.

src/nxt_application.h

@@ -103,6 +103,9 @@ struct nxt_common_app_conf_s {
     size_t                     shm_limit;
     uint32_t                   request_limit;
 
+    nxt_fd_t                   shared_port_fd;
+    nxt_fd_t                   shared_queue_fd;
+
     union {
         nxt_external_app_conf_t  external;
         nxt_python_app_conf_t    python;