COPYRIGHTS: clarify licenses more, and add license for new contributions
    
    - If new contributions are under CC0 we don't need to state that
      people also agreed to use CC0 for new contributions.
    
    - Before, I forgot to use git commit -a --amend before pushing, so
      PKGBUILDs went in instead of packages. I'm using packages to enable
      to add scripts (for instance scripts to check the licenses of the
      PKGBUILDs) under other licenses. In addition many packages have more
      files (like .install files for instance) than just PKGBUILDs and
      patches, and the other packages files are also under the CC0
      license.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>