Only read external script output when available in the main loop.

Only read external script output when available in the main loop.

Rename bar_fmt_print to bar_draw.

Change all bar_update calls to bar_draw, except in main.

Rename bar_update to bar_extra_update.
To clarify, bar_extra_update reads the external bar script output and
redraws the bar if needed; bar_draw only redraws the bar with the
current external bar script output.