MINOR: xprt: add a new show_fd() helper to complete some "show fd" dumps.

Just like we did for the muxes, now the transport layers will have the
ability to provide helpers to report more detailed information about their
internal context. When the helper is not known, the pointer continues to
be dumped as-is if it's not NULL. This way a transport with no context nor
dump function will not add a useless "xprt_ctx=(nil)" but the pointer will
be emitted if valid or if a helper is defined.
diff --git a/include/haproxy/connection-t.h b/include/haproxy/connection-t.h
index 91048d5..7dd3d15 100644
--- a/include/haproxy/connection-t.h
+++ b/include/haproxy/connection-t.h
@@ -381,6 +381,7 @@
 	int (*unsubscribe)(struct connection *conn, void *xprt_ctx, int event_type, struct wait_event *es); /* Unsubscribe <es> from events */
 	int (*remove_xprt)(struct connection *conn, void *xprt_ctx, void *toremove_ctx, const struct xprt_ops *newops, void *newctx); /* Remove an xprt from the connection, used by temporary xprt such as the handshake one */
 	int (*add_xprt)(struct connection *conn, void *xprt_ctx, void *toadd_ctx, const struct xprt_ops *toadd_ops, void **oldxprt_ctx, const struct xprt_ops **oldxprt_ops); /* Add a new XPRT as the new xprt, and return the old one */
+	void (*show_fd)(struct buffer *, const struct connection *, const void *ctx); /* append some data about xprt for "show fd" */
 };
 
 /* mux_ops describes the mux operations, which are to be performed at the
diff --git a/src/cli.c b/src/cli.c
index b737801..0603729 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -1106,8 +1106,12 @@
 				chunk_appendf(&trash, " nomux");
 
 			chunk_appendf(&trash, " xprt=%s", xprt ? xprt->name : "");
-			if (xprt)
-				chunk_appendf(&trash, " xprt_ctx=%p", xprt_ctx);
+			if (xprt) {
+				if (xprt_ctx || xprt->show_fd)
+					chunk_appendf(&trash, " xprt_ctx=%p", xprt_ctx);
+				if (xprt->show_fd)
+					xprt->show_fd(&trash, conn, xprt_ctx);
+			}
 		}
 		else if (fdt.iocb == sock_accept_iocb) {
 			chunk_appendf(&trash, ") l.st=%s fe=%s",