intl: Fix translation lookup failure when wbindtextdomain is used.

intl: Fix translation lookup failure when wbindtextdomain is used.

Reported by Luca Bacci <luca.bacci@outlook.com> at
<https://savannah.gnu.org/bugs/index.php?64311>.

The bug showed up when a call to bindtextdomain() with an absolute directory
and a call to wbindtextdomain() for a different domain was in effect.
Translations in the first referenced domain worked, translations in the second
referenced domain did not.

* gettext-runtime/intl/loadinfo.h (struct loaded_l10nfile, _nl_make_l10nflist):
Improve comments.
* gettext-runtime/intl/finddomain.c (_nl_loaded_domains): Likewise.
* gettext-runtime/intl/l10nflist.c (_nl_make_l10nflist): Fix code that looks up
the loaded_l10nfile in the given list, and make it more maintainable.
* gettext-tools/tests/intl-6: Improve comments.
* gettext-tools/tests/intl-6-prg.c (main): Restructure.
* gettext-tools/tests/intl-7: New file, based on gettext-tools/tests/intl-6.
* gettext-tools/tests/intl-6-prg.c: New file, based on
gettext-tools/tests/intl-6-prg.c.
* gettext-tools/tests/Makefile.am (TESTS): Add intl-7.
(check_PROGRAMS): Add intl-7-prg.
(intl_7_prg_SOURCES, intl_7_prg_LDADD): New variables.