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

[Daniel Brockman]

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.

-- 
Daniel Brockman <address@hidden>