stand/lua: Create a "carousel" menu entry type

This is a pre-cursor to boot environment support in lualoader. Create a new
menu item type, "carousel_entry", that generally provides a callback to get
the list of items, a carousel_id for storing the current value, and the
standard name/func functions that an entry has.

The difference between this and a normal menu item, functionally, is that
selecting a carousel item will automatically rotate through available items
and wrap back at the beginning when the list is exhausted.

The 'name' function takes the choice index, current choice, and the list of
choices as parameters so that the menu item can decorate the name freely as
desired.

The 'func' function takes the current choice as a parameter, so it can act
accordingly.

The kernel menu item has been rewritten to use the carousel_entry type as
both an example and initial test of its functionality before it is used for
boot environment options.