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.h

@@ -21,6 +21,8 @@ typedef enum {
     _NXT_PORT_MSG_DATA,
     _NXT_PORT_MSG_REMOVE_PID,
     _NXT_PORT_MSG_READY,
+    _NXT_PORT_MSG_RPC_READY,
+    _NXT_PORT_MSG_RPC_ERROR,
 
     NXT_PORT_MSG_MAX,
 
@@ -33,6 +35,9 @@ typedef enum {
     NXT_PORT_MSG_DATA_LAST      = _NXT_PORT_MSG_DATA | NXT_PORT_MSG_LAST,
     NXT_PORT_MSG_REMOVE_PID     = _NXT_PORT_MSG_REMOVE_PID | NXT_PORT_MSG_LAST,
     NXT_PORT_MSG_READY          = _NXT_PORT_MSG_READY | NXT_PORT_MSG_LAST,
+    NXT_PORT_MSG_RPC_READY      = _NXT_PORT_MSG_RPC_READY,
+    NXT_PORT_MSG_RPC_READY_LAST = _NXT_PORT_MSG_RPC_READY | NXT_PORT_MSG_LAST,
+    NXT_PORT_MSG_RPC_ERROR      = _NXT_PORT_MSG_RPC_ERROR | NXT_PORT_MSG_LAST,
 } nxt_port_msg_type_t;
 
 
@@ -104,6 +109,10 @@ struct nxt_port_s {
     nxt_port_id_t       id;
     nxt_pid_t           pid;
 
+    nxt_lvlhsh_t        rpc_streams; /* stream to nxt_port_rpc_reg_t */
+    nxt_lvlhsh_t        rpc_peers;   /* peer to queue of nxt_port_rpc_reg_t */
+    uint32_t            next_stream;
+
     nxt_process_type_t  type;
     nxt_work_t          work;
 };