CLEANUP: stream-int: consistently call the si/stream_int functions

As long-time changes have accumulated over time, the exported functions
of the stream-interface were almost all prefixed "si_<something>" while
most private ones (mostly callbacks) were called "stream_int_<something>".
There were still a few confusing exceptions, which were addressed to
follow this shcme :
  - stream_sock_read0(), only used internally, was renamed stream_int_read0()
    and made static
  - stream_int_notify() is only private and was made static
  - stream_int_{check_timeouts,report_error,retnclose,register_handler,update}
    were renamed si_<something>.

Now it is clearer when checking one of these if it risks to be used outside
or not.
diff --git a/src/cli.c b/src/cli.c
index cc377fc..fba7513 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1749,7 +1749,7 @@
 	struct buffer *buf = get_trash_chunk();
 
 	chunk_initstr(buf, msg);
-	stream_int_retnclose(&s->si[0], buf);
+	si_retnclose(&s->si[0], buf);
 }
 
 static enum obj_type *pcli_pid_to_server(int proc_pid)