(splash-screen-keymap): Rename from `fancy-splash-keymap'

(splash-screen-keymap): Rename from `fancy-splash-keymap'
because it's common to both types of splash screen: fancy and normal.
Bind SPC to scroll-up, DEL to scroll-down and `q' to exit-splash-screen.
(exit-splash-screen): Rename from `fancy-splash-quit'.
Use `quit-window' instead of `kill-buffer'.
(fancy-splash-head): Use make-button to insert GNU image link.
(fancy-splash-screens, normal-splash-screen): Rename " About GNU
Emacs" to "*About GNU Emacs*", and " GNU Emacs" to "*GNU Emacs*".
(normal-splash-screen): Put "Browse manuals" on the same line with
"Emacs manual".  Remove descriptions from "Useful tasks" and put
all links in two columns on two lines.