bug#6671: Avoid recentering when user says so

[David Engster]

Eli Zaretskii writes:
> Would people who use these variables please try the patch below, and
> report any findings, both positive and negative?  Please report your
> findings to the bug tracker at 6671@debbugs.gnu.org, so that the
> details get filed there.
>
> Given positive feedback, I will commit this to the trunk.

Oh, please do! Using this patch, scrolling is definitely *much* improved
with my setup. No more annoying recenterings - Thank you!

-David