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

[Ryan Yeske]

   In principle, there is only one overlay-arrow-position variable,
   and since its value is a marker, its value includes the buffer
   in which it occurs.

   This is how e.g. gdb can easily move the overlay arrow between buffers
   -- just modify the global variable.  

gdb could use a *gud* buffer local variable.  That way multiple
instances of gud wouldn't steal the arrow from eachother.