bug#27091: Request: Toggle scroll bar on each window separately

[martin rudalics]

> The above are just fine.  The problem is on the other end: the
> set-window-scroll-bars function doesn't mention that switching buffers
> in the window for which that function was called will reinstate the
> scroll bar.  (It also doesn't say what is the meaning of
> VERTICAL-TYPE's nil value.)  And neither does the manual.

I tried to fix these.  Please have a look.

martin