MEDIUM: stream-int: replace channel_alloc_buffer() with si_alloc_ibuf() everywhere

Well that's only 3 places (applet.c, stream_interface.c, hlua.c). This
ensures we always clear SI_FL_WAIT_ROOM before setting it on failure,
so that it is granted that SI_FL_WAIT_ROOM always indicates a lack of
room for doing an operation, including the inability to allocate a
buffer for this.
diff --git a/src/hlua.c b/src/hlua.c
index 44fb04a..6ad3c82 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -2030,7 +2030,7 @@
 	 * the request buffer if its not required.
 	 */
 	if (s->req.buf.size == 0) {
-		if (!channel_alloc_buffer(&s->req, &appctx->buffer_wait))
+		if (!si_alloc_ibuf(si, &appctx->buffer_wait))
 			goto hlua_socket_write_yield_return;
 	}