(initial-buffer-choice): Rename choice "Splash screen"

(initial-buffer-choice): Rename choice "Splash screen"
to "Startup screen".  Fix docstring.
(inhibit-startup-screen): Rename from `inhibit-splash-screen'.
(inhibit-splash-screen): Make alias to `inhibit-startup-screen'.
(inhibit-startup-message): Change alias to `inhibit-startup-screen'.
(initial-scratch-message): Fix docstring.
(fancy-startup-text): Move link to Emacs Manual below Emacs Guided
Tour (which is a kind of tutorial and will be next to Emacs Tutorial).
Add link to "Customize Startup" and set interval between links to
5 spaces.
(fancy-about-text): Add links "Authors" and "Contributing".
(fancy-splash-head): Add text "Welcome to " on the startup screen,
and "This is " on the about screen.  Add link to
"http://www.gnu.org/software/emacs/" for "GNU Emacs".
For the about screen move emacs version to the header from
`fancy-splash-tail' (as it's done already for normal about screen).
(fancy-splash-tail): Insert emacs version only for startup screen.
(normal-splash-screen): Remove duplicate empty lines.
(normal-about-screen): Add links "Authors" and "Contributing".