Fix follow-scroll-up/down, making them replacements for scroll-up/down.

Fix follow-scroll-up/down, making them replacements for scroll-up/down.

1. Allow point to move between follow windows in scroll operations.
2. Fix bug where `right-char' just before EOB caused spurious scrolling,
when EOB was isolated in the last follow window.

lisp/follow.el (follow-fixed-window): New variable.
(follow-get-scrolled-point): New function.
(follow-scrol-up, follow-scroll-down): Add autoload cookies.
Reformulate
the code.  Put `scroll-command' properties on the functions.  Correct
minor errors in ...-down's doc string and code.
(follow-calc-win-end): Amend incomplete doc string.  Use
`pos-visible-in-window-p' to check whether EOB is in the window.
(follow-estimate-first-window-start): Correct an off-by-1 error.
(follow-adjust-window): Add handling for explicit scrolling operations.