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_master_process.c

@@ -200,6 +200,8 @@ static nxt_port_handler_t  nxt_master_process_port_handlers[] = {
     nxt_port_master_data_handler,
     NULL,
     nxt_port_ready_handler,
+    nxt_port_rpc_handler,
+    nxt_port_rpc_handler,
 };