Use new approach for gnulib localizations.

Use new approach for gnulib localizations.

* gettext-runtime/po/POTFILES.in: Remove files that come from gnulib.
* gettext-tools/po/POTFILES.in: Likewise.
* gettext-runtime/src/envsubst.c (main): Invoke bindtextdomain for the domain
'gnulib'.
* gettext-runtime/src/gettext.c (main): Likewise.
* gettext-runtime/src/ngettext.c (main): Likewise.
* gettext-tools/src/cldr-plurals.c (main): Likewise.
* gettext-tools/src/hostname.c (main): Likewise.
* gettext-tools/src/msgattrib.c (main): Likewise.
* gettext-tools/src/msgcat.c (main): Likewise.
* gettext-tools/src/msgcmp.c (main): Likewise.
* gettext-tools/src/msgcomm.c (main): Likewise.
* gettext-tools/src/msgconv.c (main): Likewise.
* gettext-tools/src/msgen.c (main): Likewise.
* gettext-tools/src/msgexec.c (main): Likewise.
* gettext-tools/src/msgfilter.c (main): Likewise.
* gettext-tools/src/msgfmt.c (main): Likewise.
* gettext-tools/src/msggrep.c (main): Likewise.
* gettext-tools/src/msginit.c (main): Likewise.
* gettext-tools/src/msgmerge.c (main): Likewise.
* gettext-tools/src/msgunfmt.c (main): Likewise.
* gettext-tools/src/msguniq.c (main): Likewise.
* gettext-tools/src/recode-sr-latin.c (main): Likewise.
* gettext-tools/src/urlget.c (main): Likewise.
* gettext-tools/src/xgettext.c (main): Likewise.