gettext, ngettext: Fix the expansion of '\\' and octal escape sequences.
* gettext-runtime/src/gettext.c (expand_escape): Don't swallow the expansion
of '\\' and octal escape sequences.
* gettext-runtime/src/ngettext.c (expand_escape): Likewise.
* gettext-tools/tests/tstgettext.c (expand_escape): Likewise.
* NEWS: Mention the change.
