msgmerge, msginit: Reject POT files with non-ASCII characters, except in UTF-8.

msgmerge, msginit: Reject POT files with non-ASCII characters, except in UTF-8.

Reported by Martin Quinson <mquinson@debian.org>
at <https://savannah.gnu.org/bugs/?65104>.

* gettext-tools/src/msgl-charset.h (check_pot_charset): New declaration.
* gettext-tools/src/msgl-charset.c: Include msgl-ascii.h.
(check_pot_charset): New function.
* gettext-tools/src/msginit.c: Include msgl-charset.h.
(main): Invoke check_pot_charset.
* gettext-tools/src/msgmerge.c: Include msgl-charset.h.
(merge): Invoke check_pot_charset.
* gettext-tools/src/FILES: Update.
* gettext-tools/tests/testdata/nonascii.pot: New file.
* gettext-tools/tests/msginit-5: New file.
* gettext-tools/tests/msgmerge-charset-4: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.
(EXTRA_DIST): Add testdata/nonascii.pot.