MINOR: stream-int/stream: Move si_retnclose() in the stream scope

si_retnclose() is used to send a reply to a client before closing. There is
no use on the server side, in spite of the function is generic. Thus, it is
renamed stream_retnclose() and moved into the stream scope. The function now
handle a stream and explicitly send a message to the client.
diff --git a/src/cli.c b/src/cli.c
index 5ed96f5..baea805 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -2200,7 +2200,7 @@
 	struct buffer *buf = get_trash_chunk();
 
 	chunk_initstr(buf, msg);
-	si_retnclose(cs_si(s->csf), buf);
+	stream_retnclose(s, buf);
 }
 
 static enum obj_type *pcli_pid_to_server(int proc_pid)