bug#25308: Shorten long "ui-lines" in Custom buffers

[Jonas Bernoulli <jonas@bernoul.li>]

Custom buffers contain two long lines, the first separating the initial
buttons from the options shown below, and the second is for symmetry, I
suppose.  This is implemented in `custom-group-value-create'.

These lines are 999 characters long.  When point ends up on the "line
line", then it ends up at the *end* of that line, scrolling all content
except for the line itself off-window.  This happens both with C-n/C-p,
as well as when using the mouse scroll wheel.

This is very distracting and I would suggest that these lines be removed
completely.  If that is not an option, then I would suggest to make the
lines short enough for them to almost always end before the window edge.
80 characters instead of 999 would do, I think.

If that is considered to be too ugly, then I would suggest setting the
`:align-to' to be just long enough to reach the edge of the window (but
never longer) at the time `custom-group-value-create' is called, using
something like:

(list 'space :align-to
      `(+ (0 . right)
          ,(min (window-hscroll)
                (- (line-end-position)
                   (line-beginning-position)))))

Thanks for considering these options.