msgfmt: Fix -k/--keyword option handling.

msgfmt: Fix -k/--keyword option handling.

Reported by Daniel <daniel.rf710@gmail.com> in
<https://savannah.gnu.org/bugs/index.php?63455>.

* gettext-tools/src/msgfmt.c (long_options): Declare that --keyword takes an
optional argument.
(main): Recognize -k as an alternative to --keyword.
Ensure that desktop_keywords is initialized before use.
Don't set desktop_default_keywords to false when an option -kKWSPEC or
--keyword=KWSPEC is seen.
Don't destroy the desktop_keywords table too early.
Don't destroy the desktop_keywords table when the program is terminating anyway.