xgettext: Fix abort when outputting a msgid that has an invalid UTF-8 character.

xgettext: Fix abort when outputting a msgid that has an invalid UTF-8 character.

* gettext-tools/src/xg-encoding.c: Include unistr.h.
(non_utf8_error_message): New function.
(from_current_source_encoding): When xgettext_current_source_encoding is
"UTF-8", check that the string is well-formed UTF-8.
* gettext-tools/tests/xgettext-c-8: New file.
* gettext-tools/tests/xgettext-python-5: New file.
* gettext-tools/tests/xgettext-elisp-3: New file.
* gettext-tools/tests/xgettext-librep-3: New file.
* gettext-tools/tests/xgettext-awk-3: New file.
* gettext-tools/tests/xgettext-lua-3: New file.
* gettext-tools/tests/xgettext-vala-4: New file.
* gettext-tools/tests/xgettext-php-5: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add them.