hash: switch to CC0 license.

As per email from Andreas (the only other contributor):
From: Andreas Schlick <schlick@lavabit.com>
Subject: Re: Changing from Public Domain to CC0
To: Rusty Russell <rusty@rustcorp.com.au>
Date: Thu, 01 Nov 2012 12:51:47 +0100

Hi!

> Since Public Domain is a slippery concept outside certain countries,
> it has been suggested that I formalize it to Creative Commons Zero.
>
> I want to change the License string (and add a symlink) to the
> following trivial modules.  Please Ack.

Sure, ack.

 Andreas

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/hash/LICENSE

@@ -0,0 +1 @@
+../../licenses/CC0

ccan/hash/_info

@@ -13,7 +13,7 @@
  * The stable ones will always give the same results on any computer,
  * and on any version of this package.
  *
- * License: Public domain
+ * License: CC0 (Public domain)
  * Maintainer: Rusty Russell <rusty@rustcorp.com.au>
  * Author: Bob Jenkins <bob_jenkins@burtleburtle.net>
  */

ccan/hash/hash.c

@@ -1,3 +1,4 @@
+/* CC0 (Public domain) - see LICENSE file for details */
 /*
 -------------------------------------------------------------------------------
 lookup3.c, by Bob Jenkins, May 2006, Public Domain.