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/proto_htx.c b/src/proto_htx.c
index 86bf38e..4573c68 100644
--- a/src/proto_htx.c
+++ b/src/proto_htx.c
@@ -548,7 +548,7 @@
 	 */
 	if (htx_stats_check_uri(s, txn, px)) {
 		s->target = &http_stats_applet.obj_type;
-		if (unlikely(!stream_int_register_handler(&s->si[1], objt_applet(s->target)))) {
+		if (unlikely(!si_register_handler(&s->si[1], objt_applet(s->target)))) {
 			txn->status = 500;
 			s->logs.tv_request = now;
 			htx_reply_and_close(s, txn->status, htx_error_message(s));