'_emit_areas_line' returns 1 for success and 0 for failure. This always

'_emit_areas_line' returns 1 for success and 0 for failure.  This always
confuses me, so I've added a comment at the top of the function to
remind me of this.

I also found that 'mirror_emit_segment_line' was returning 0 (return_0)
on failure /and/ success.  It now returns 1 for success and 0 for failure -
just like '_emit_areas_line' and its calling function, '_emit_segment_line'.