Create and use sbbs_t::menu_exists() - solves a couple of long-standing issues:

- if the sysop had text/menu/<filename>.bak (for example), the optoinal menu
  would attempt to load, resulting in an error ("<filename>.asc not found")
- if the sysop specified menu_file or menu_dir (e.g. via Baja), the existing
  checks for optional menu file existance would not work