Re: `overlay-arrow-position' should be automatically buffer-local

[Ryan Yeske <rcyeske@gmail.com>]

   I believe the variable `overlay-arrow-position' should
   automatically become buffer-local whenever it is set.

   It appears that Gnus fails to do this manually, instead
   ending up setting the global value to a marker that refers
   to the Gnus summary buffer, which doesn't make sense.

   In fact, I see no reason to ever set the global value of
   this variable.  In any case, it should not be the default.

I recently noticed when adding an overlay arrow to rcirc that making
it `overlay-arrow-position' buffer local *does* work (you can get an
arrow in multiple buffers), even though (elisp)Overlay Arrow says:

 - Variable: overlay-arrow-position

     [...]

     The overlay string is displayed only in the buffer that this marker
     points into.  Thus, only one buffer can have an overlay arrow at
     any given time.

Which I think was true in 21.