Clarify copyright and license of some files.
Reported by Joël Krähemann at <https://savannah.gnu.org/bugs/?52227>.
* gettext-runtime/po/Makefile.in.in: Correct copyright holder.
* gettext-runtime/po/remove-potcdate.sin: Use the same licensing terms as Makefile.in.in.
* gettext-runtime/po/Rules-quot: Use an all-permissive license.
* gettext-tools/po/Makevars.template: Likewise.
* gettext-runtime/po/Makevars: Likewise.
* gettext-tools/po/Makevars: Likewise.
* gettext-runtime/po/POTFILES.in: Use GPLv3+.
* gettext-tools/po/POTFILES.in: Likewise.
