bug#34513: display-line-numbers in term mode

[Eli Zaretskii]

> From: Stefan Monnier <monnier@IRO.UMontreal.CA>
> Date: Wed, 27 Feb 2019 10:26:04 -0500
> Cc: 34513@debbugs.gnu.org
> 
> PS: Why is the function called `line-number-display-width` when
> everything else seems to use the `display-line-number` prefix?  

Because display-line-numbers-line-number-display-width is a mouthful
that I couldn't live with, and also repeats 3 words twice each.

> -        (ncols (/ window-width font-width)))
> +        (ncols (- (/ window-width font-width) (line-number-display-width))))

I think you want (line-number-display-width 'columns), since this
wants the canonical column units, right?