po: Add customizable 'fetch-po' script.

po: Add customizable 'fetch-po' script.

* gettext-tools/wizard/po-templates/traditional/fetch-po: New file.
* gettext-tools/wizard/Makefile.am (po_SCRIPTS): New variable.
(EXTRA_DIST): Add the po_SCRIPTS.
* gettext-tools/wizard/gettextize.in: Copy but don't overwrite po/fetch-po.
* gettext-tools/wizard/po-templates/traditional/Makefile.in.in (fetch-po): New
target.
* gettext-tools/examples/po/Makefile.am (fetch-po): New target.
* gettext-tools/examples/hello-*/po/Makefile.am (fetch-po): New target.
* gettext-tools/examples/hello-objc-gnustep/po/GNUmakefile (fetch-po): New
target.
* gettext-tools/doc/gettext.texi (po/fetch-po): New subsection.
(po/LINGUAS): Explain how it related to the 'fetch-po' script.
(Release Management): Document how to fetch the PO files before a release.
* NEWS: Mention the change.