Add CC0 as the license. SPDX does not have a "public domain" tag, but CC0 is
more or less equivalent. We should have *some* header to avoid doubts in the
future.
same motivation as in #5816:
- distributions have scripts to rewrite shebangs on installation and
they know what locations to rely on.
- For tests/compilation we should rather rely on the user to have setup
there PATH correctly.