lualoader: unload upon kernel change if a kernel was previously loaded

In the majority of cases, a kernel is not loaded before we hit the menu.
However, if a password is set, we'll trigger autoboot and have loadelf'd
beforehand. We also need to take into account one dropping to the loader
prompt and twiddling with things manually; if they try to toggle through
kernels, we'll assume they mean it.

Reported by: trasz
MFC after: 3 days