BUG/MEDIUM: cli/debug: Properly get the stream-int in all debug I/O handlers

The appctx owner is not a stream-interface anymore. It is now a conn-stream.
In the cli I/O handler for the command "debug dev fd", we still handle it as
a stream-interface. It is now fixed.

It is 2.6-specific, no backport is needed.
diff --git a/src/debug.c b/src/debug.c
index ebdbb88..73fe159 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -1040,7 +1040,7 @@
  */
 static int debug_iohandler_fd(struct appctx *appctx)
 {
-	struct stream_interface *si = appctx->owner;
+	struct stream_interface *si = cs_si(appctx->owner);
 	struct sockaddr_storage sa;
 	struct stat statbuf;
 	socklen_t salen, vlen;