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

[Drew Adams]

> > What's unclear about, say, `line-number-width'?
> 
> It's not unclear, it's wrong.

I didn't realize that this is a minor-mode global
function.

Emacs convention says that the names of global
symbols (such as this) should start with either
the mode name OR an abbreviation.

The mode name `display-line-numbers' (mode) is
long enough to deserve an abbreviation for use
as prefix.  Please consider coming up with one.

Maybe something like `linenum-' or `line-number-'.

In that case, this function could be called just
`linenum-width' or `linenum-number-width'.