ui: Make '--version' output GCS-compliant.

* guix/ui.scm (show-version-and-exit): Display copyright year, license,
  and LACK OF WARRANTY.

guix/ui.scm

@@ -137,6 +137,11 @@ messages."
   "Display version information for COMMAND and `(exit 0)'."
   (simple-format #t "~a (~a) ~a~%"
                  command %guix-package-name %guix-version)
+  (display (_ "Copyright (C) 2013 the Guix authors
+License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
+This is free software: you are free to change and redistribute it.
+There is NO WARRANTY, to the extent permitted by law.
+"))
   (exit 0))
 
 (define (show-bug-report-information)