Avoid the need for charset conversion at run time on musl libc.

Avoid the need for charset conversion at run time on musl libc.

This fixes 1 system-tests failure (intl-2) and 3 tests failures (lang-c,
lang-c++, lang-sh).

* gettext-tools/src/write-mo.h (no_convert_to_utf8): New declaration.
(msgdomain_write_mo): Add a 4th parameter.
* gettext-tools/src/write-mo.c: Include po-charset.h, msgl-iconv.h.
(no_convert_to_utf8): New variable.
(msgdomain_write_mo): Add a 4th parameter. Convert the message list to UTF-8.
* gettext-tools/src/msgfmt.c (long_options): Add --no-convert.
(main): Handle the --no-convert option. Pass a 4th argument to
msgdomain_write_mo.
(usage): Document the --no-convert option.
* gettext-tools/doc/msgfmt.texi: Document the --no-convert option.
* gettext-tools/doc/gettext.texi (MO Files): Document the character encoding of
strings in MO files.
* gettext-tools/tests/msgfmt-5: Use msgfmt option --no-convert.
* gettext-tools/tests/msgfmt-12: Likewise.
* gettext-tools/tests/lang-pascal: Likewise.
* NEWS: Mention the change.