lualoader: Address some 'luacheck' concerns

luacheck pointed out an assortment of issues, ranging from non-standard
globals being created as well as unused parameters, variables, and redundant
assignments.

Using '_' as a placeholder for values unused (whether it be parameters
unused or return values unused, assuming multiple return values) feels clean
and gets the point across, so I've adopted it. It also helps flag candidates
for cleanup later in some of the lambdas I've created, giving me an easy way
to re-evaluate later if we're still not using some of these features.