bug#32849: 26.1; xref-marker-ring-length user option doesn't have setter

[Eli Zaretskii]

> From: Allen Li <darkfeline@felesatra.moe>
> Date: Wed, 26 Sep 2018 16:14:15 -0700
> 
> The xref-marker-ring-length user option doesn't have a setter, so
> customizing it will not affect xref--marker-ring, except if/when user
> customizations are loaded in a new Emacs process before xref is loaded.
> Customizations after xref is loaded will have no effect.

I think the setter should use ring-extend to enlarge the ring, and
some custom code using ring-remove to make the ring smaller.

Would you like to submit a patch along those lines?

Thanks.