menuconfig: remember alternate config filename

When loading an alternate configuration use that file as
current configuration filename.
Make the filename visible in the dialog.
Default continue to be .config.

Inspired by patch from: Cyrill Gorcunov <gorcunov@gmail.com>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc:  Cyrill Gorcunov <gorcunov@gmail.com>

scripts/kconfig/lxdialog/dialog.h

@@ -188,6 +188,7 @@ int on_key_esc(WINDOW *win);
 int on_key_resize(void);
 
 void init_dialog(const char *backtitle);
+void set_dialog_backtitle(const char *backtitle);
 void reset_dialog(void);
 void end_dialog(void);
 void attr_clear(WINDOW * win, int height, int width, chtype attr);