Port RPC interface introduced.

Usage:
1. Register handlers in incoming port with nxt_port_rpc_register_handler().
2. Use return value as a stream identifier for next nxt_port_socket_write().

src/nxt_router.c

@@ -1407,6 +1407,9 @@ static nxt_port_handler_t  nxt_router_app_port_handlers[] = {
     nxt_port_mmap_handler,
     nxt_router_app_data_handler,
     nxt_port_remove_pid_handler,
+    NULL,
+    nxt_port_rpc_handler,
+    nxt_port_rpc_handler,
 };