lualoader: A little more general menu cleanup

Instead of a single-letter parameter ('m'), use something a little more
descriptive and meaningful: 'menudef' ("menu definition") -- these functions
expect to be passed a menudef, so call it what it is.

While here, throw an assertion in that we have a handler for the selected
menu item. This is more of a debugging aide so that it's more obvious when
one is testing a menudef that they've added an entry item that we don't
handle.

This is an improvement over the past behavior of ignoring the unknown menu
entry.