lualoader: Track the menu currently drawn, instead of validity

This cleans up the odd approach to menu drawing. Instead of tracking
validity, we track the menu that was drawn on the screen. Whenever we draw a
menu, we'll set this to that menu.

Anything that invalidates the screen should go ahead and trigger an explicit
redraw, rather than finding a wy to set screen_invalid.

The currently drawn menu is then reset in menu.run as we exit the menu
system, so that dropping to the loader prompt or leaving menu.run() will
just behave as expected without doing redundant work every time we leave a
menu.