xgettext: In language PO, handle non-ASCII non-UTF-8 input files correctly.

xgettext: In language PO, handle non-ASCII non-UTF-8 input files correctly.

* gettext-tools/src/x-po.c: Include msgl-iconv.h, msgl-ascii.h, po-charset.h.
(extract): If a header charset is present, convert the messages to UTF-8;
otherwise verify that they are all ASCII.
* gettext-tools/tests/xgettext-po-3: New file.
* gettext-tools/tests/xgettext-po-4: New file.
* gettext-tools/tests/testdata/xg-po-3.po: New file.
* gettext-tools/tests/testdata/xg-po-4.po: New file.
* gettext-tools/tests/Makefile.am (TESTS, EXTRA_DIST): Add them.
* NEWS: Mention the change.