bug#2145: 23.0.90; S-SPC should scroll up in view-only buffers

[Rudi Schlatte <rudi@constantly.at>]

(This should be filed as "wishlist")

Summary:

S-SPC should scroll up in view-only buffers, such as info, gnus
messages and buffers in view-mode; currently it scrolls down.

Rationale:

Most other programs that display more than a screenful of information
(Firefox, various mail clients, the evince pdf viewer, ...) use SPC to
scroll down and S-SPC (Shift-Space) to scroll up.  Emacs uses SPC (and
S-SPC) to scroll down and <backspace> to scroll up.  Additionally, in
some programs, Backspace is bound to some deleting action (in my email
reader, it moves the current message to Trash).  Paging up and down
through stuff thus requires me to be careful about which program I'm
in; I've accidentally deleted emails after reading too much stuff in
emacs.

Workarounds:

Putting the appropriate define-key forms, e.g.

(define-key view-mode-map (kbd "S-SPC") 'View-scroll-page-backward)
(define-key Info-mode-map (kbd "S-SPC") 'Info-scroll-down)

in the user's init file.