(fancy-startup-tail): Say exactly what does the button

(fancy-startup-tail): Say exactly what does the button
dismiss ("Dismiss this startup screen").  Use text "Never show
it again" for the checkbox after this button.
(fancy-startup-screen, fancy-about-screen): Put point before the
first link, so the user can quickly select links with the keyboard.
(normal-mouse-startup-screen): Add more useful text describing how
to follow a link.