Update license headers

[jmwilson]

• Add CC0 license text file
• Set CC0 as the "license" for trivial files: configuration, metadata, Makefiles. This is REUSE's recommendation for excluding files from licensing. codeql.yml was missing the header making the project non-compliant.
• Replace FSFUL with CC0, since CC0 covers the grant for the INSTALL file: "the Free Software Foundation gives
  unlimited permission to copy, distribute and modify it"
• autogen.sh: shell script shebang must be the first line, #!/bin/sh is correct for any POSIX system.
• update some lines in README around OS support