WIP: Update copyright statements

Approver's checklist:

• The author has submitted the FSF Copyright Assignment and is listed in AUTHORS
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with existing code
• This feature/change has adequate documentation added (if appropriate)
• No obvious mistakes / misspelling in the code

Pending:

• Find all files missing a license and add one
• Write a script / figure out how to use gnulib's scripts to ensure we never miss a file again