enforce a slightly more sensible default header

enforce a slightly more sensible default header

In 12f7536 I chose to opt for a slightly better text which avoids
assigning copyright to the FSF (not because it would be wrong to do it,
but is usually not what the contributor intended, but just the default.

xgettext has a --foreign flag for avoiding the copyright, but po4a
hasn't and an empty copyright-holder doesn't work either, so this little
rework of files with sed and cat.

Git-Dch: Ignore