MINOR: cli/show_fd: report that a connection is back or not

It simply adds a "back={0,1}" depending on the connection's side, it's
useful to debug.
diff --git a/src/cli.c b/src/cli.c
index fba7513..687796f 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -888,6 +888,7 @@
 		const struct mux_ops *mux = NULL;
 		void *ctx = NULL;
 		uint32_t conn_flags = 0;
+		int is_back = 0;
 
 		fdt = fdtab[fd];
 
@@ -901,6 +902,7 @@
 			li = objt_listener(((struct connection *)fdt.owner)->target);
 			sv = objt_server(((struct connection *)fdt.owner)->target);
 			px = objt_proxy(((struct connection *)fdt.owner)->target);
+			is_back = conn_is_back((struct connection *)fdt.owner);
 		}
 		else if (fdt.iocb == listener_accept)
 			li = fdt.owner;
@@ -936,7 +938,7 @@
 			     "unknown");
 
 		if (fdt.iocb == conn_fd_handler) {
-			chunk_appendf(&trash, " cflg=0x%08x", conn_flags);
+			chunk_appendf(&trash, " back=%d cflg=0x%08x", is_back, conn_flags);
 			if (px)
 				chunk_appendf(&trash, " px=%s", px->id);
 			else if (sv)