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_port_rpc.h

@@ -0,0 +1,24 @@
+
+/*
+ * Copyright (C) Max Romanov
+ * Copyright (C) NGINX, Inc.
+ */
+
+#ifndef _NXT_PORT_RPC_H_INCLUDED_
+#define _NXT_PORT_RPC_H_INCLUDED_
+
+
+typedef void (*nxt_port_rpc_handler_t)(nxt_task_t *task,
+    nxt_port_recv_msg_t *msg, void *data);
+
+uint32_t nxt_port_rpc_register_handler(nxt_task_t *task, nxt_port_t *port,
+    nxt_port_rpc_handler_t ready_handler, nxt_port_rpc_handler_t error_handler,
+    nxt_pid_t peer, void *data);
+void nxt_port_rpc_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg);
+void nxt_port_rpc_remove_peer(nxt_task_t *task, nxt_port_t *port,
+    nxt_pid_t peer);
+void nxt_port_rpc_cancel(nxt_task_t *task, nxt_port_t *port, uint32_t stream);
+
+
+#endif /* _NXT_PORT_RPC_H_INCLUDED_ */
+