This is driven by an urge to separate out the bits that really only need to
happen when the menu system starts up. Key points:
- menu.process now does the bulk of menu handling. It retains autoboot
handling for dubious reasons, and it no longer accepts a 'nil' menu to
process as 'the default'. Its return value is insignificant.
- The MENU_SUBMENU handler now returns nothing. If menu.process has exited,
then we continue processing menu items on the parent menu as expected.
- menu.run is now the entry point of the menu system. It checks whether the
menu should be skipped, processes the default menu, then returns.
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