MFC r344243, r345517-r345518: lualoader: More intelligent screen clearing

r344243:
lualoader: only clear the screen before first password prompt

This was previously an unconditional screen clear, regardless of whether or
not we would be prompting for any passwords. This is pointless, given that
the screen clear is only there to put our screen into a consistent state
before we draw the prompts and do cursor manipulation.

This is also the only screen clear besides that to draw the menu. One can
now see early pre-loader and loader output with the menu disabled, which may
be useful for diagnostics.

r345517:
lualoader: Clear the screen before prompting for password

Assuming that the autoboot sequence was interrupted, we've done enough
cursor manipulation that the prompt for the password will be sufficiently
obscured a couple of lines up. Clear the screen and reset the cursor
position here, too.

r345518:
lualoader: Fix up some luacheck concerns

• Garbage collect an unused (removed because it was useless) constant
• Don't bother with vararg notation if args will not be used