This is a modified conio. It doesn't draw a cursor at all.

More particularly, the standard cc65 conio always draws a cursor, whether
the cursor is enabled or not. When it's disabled, the cursor is drawn
as a space character.

Benefits of doing this:

- Faster and smoother screen updates
- Slightly smaller code (20-30 bytes)
- No more "cursor ghosts" at timed prompts
- No need for a hide_cursor() function

Disadvantage: Have to draw the cursor myself when needed. It turns out
that only agetc() needs to do this, so not a big deal. Also had to
add an agetc_no_cursor() for timed_getch() to use.