lualoader: Add a twiddle at password prompt

This gives some form of feedback while typing, and matches-(ish*) Forth
behavior. The cursor generally rests two column after the password prompt,
then the twiddle is drawn three columns later and the cursor reset to
resting position after being drawn.

I've removed the note about re-evaluating it for security considerations and
instead set it up as a module-local variable that we can set later depending
on environment or something. It's set to false with no chance of changing at
the moment.

*As close as I can tell from reading check-password.4th, because I don't
have an easy test (or deployed) setup for forth loader to check how close
it is. Please do mention if it's not close enough.