Reset sbbs_t menu path vars (menu_dir and menu_file) before execing cmd shell

I obvserved an issue with Amessyroom's bbs where his custom (JS) shell is
setting bbs.menu_dir but when switching to another stock shell, his menu
files were still being displayed (with mismatching command keys as a result).

bbs.menu_dir (and implicitly, the set_menu_dir Baja function) apparently have
not been used in this way (much) before.