bug#6933: 24.0.50; fringe-mode value of `half' is broken

[Eli Zaretskii <eliz@gnu.org>]

> From: "Drew Adams" <drew.adams@oracle.com>
> Date: Fri, 27 Aug 2010 17:16:26 -0700
> 
> emacs -Q
>  
> M-x fringe-mode
> Enter `half'.
>  
> The fringe values (both left and right) should each be 5, according to
> the code and comments.  Instead, it is still 8, as shown by
> (frame-parameters).  There is no difference in behavior between `half'
> and `default'.  Dunno whether this is Windows-specific.

This happens because fringe.el has this customization form for "half"
in the defcustom for fringe-mode:

		 (const :tag "Half width" (5 . 5))

And the doc string for fringe-mode says this, among the rest:

                               This command may round up the left and
    right width specifications to ensure that their sum is a multiple
    of the character width of a frame.

So 5 is rounded up to 8, and the rest is history.

I don't know why the values are 5 instead of 4.  The default fringe
width is 8 on all GUI displays, so half of that should have been 4,
not 5.  Maybe I'm missing some subtlety here.

Can someone please check on X whether (fringe-mode '(4 . 4)) does TRT?
If it does, I think we should use 4 instead of 5 in the above
defcustom.