MINOR: connection: Add macros to know if a conn or a cs uses an HTX mux

IS_HTX_CONN() and IS_HTX_CS may now be used to know if a connection or a
conn-stream use an HTX based multiplexer.
diff --git a/include/proto/connection.h b/include/proto/connection.h
index 30730f0..97c7899 100644
--- a/include/proto/connection.h
+++ b/include/proto/connection.h
@@ -39,6 +39,9 @@
 extern struct xprt_ops *registered_xprt[XPRT_ENTRIES];
 extern struct mux_proto_list mux_proto_list;
 
+#define IS_HTX_CONN(conn) ((conn)->mux && ((conn)->mux->flags & MX_FL_HTX))
+#define IS_HTX_CS(cs)     (IS_HTX_CONN((cs)->conn))
+
 /* I/O callback for fd-based connections. It calls the read/write handlers
  * provided by the connection's sock_ops.
  */