Since this repository has multi-licensed code, add GPL v2+ file headers
to code that had no licensing blocks to make it obvious which files
carry which license.
And add "AND GPL-3.0-or-later" to extension.json's license-name property
to make it clear that this extension does have code that isn't
redistributable under GPL v2.
Change-Id: Id3059fb9596527ef054bec9d89a28f1ccbe2113d