Tweak link and startup screen faces.

Tweak link and startup screen faces.

See http://lists.gnu.org/archive/html/emacs-devel/2011-07/msg00478.html
for a discussion of the link face.  The changes to the startup faces
are so that they can take advantage of the light/dark background
settings already defined for those faces.

* lisp/faces.el (link): Use a less saturated blue on light backgrounds.

* lisp/startup.el (fancy-startup-text, fancy-about-text)
(fancy-startup-tail): Use font-lock faces, for background safety.