REORG/MINOR: stream_interface: move si->fd to struct connection

The socket fd is used only when in socket mode and with a connection.
diff --git a/src/frontend.c b/src/frontend.c
index d2a9e70..63a2c1f 100644
--- a/src/frontend.c
+++ b/src/frontend.c
@@ -52,7 +52,7 @@
  */
 int frontend_accept(struct session *s)
 {
-	int cfd = s->si[0].fd;
+	int cfd = si_fd(&s->si[0]);
 
 	tv_zero(&s->logs.tv_request);
 	s->logs.t_queue = -1;