MFC r339677-r339678, r339702, r339805: Lualoader Bugfixes

r339677:
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.

r339678:
menu.lua: Abort autoboot sequence on failed command

Currently, a timeout in the menu autoboot sequence would effectively do
nothing. We would return from the autoboot handling, then begin processing
the menu without redrawing it.

This change makes the behavior a little more friendly. Returning the user to
the menu can't have any good effects, so abort the autoboot sequence and
drop to the loader prompt.

r339702:
lualoader: Improve module loading diagnostics

Some fixes:

• Maintain historical behavior more accurately w.r.t verbose_loading; verbose_loading strictly prints "${module_name...}" and later "failed!" or "ok" based on load success
• With or without verbose_loading, dump command_errbuf on load failure. This usually happens prior to ok/failed if we're verbose_loading

r339805:
lualoader: Always return a proper dictionary for blacklist

If module_blacklist isn't specified, we have an empty blacklist; effectively
the same as if module_blacklist="" were specified in loader.conf(5).

This was reported when switching to a BE that predated the module_blacklist
introduction, but the problem is valid all the same and likely to be tripped
over in other scenarios.

Approved by: re (kib)