lualoader: only clear the screen before first password prompt

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.

Reported by:	ian
MFC after:	3 days