Update public domain mark, using text from CC0

A crisp and concise statement on copyright restrictions shows care for the
code, which in turn inspires confidence and encourages reuse. I adapted the
public domain mark from the text of the CC0 license. I also dropped Asher's
credit.

Change-Id: Icdc69e4e5fdc611c1309056c1e87d1cade0abcdd