xgettext: Deprecate option '--sort-output'.
Suggested by Lauri Nurmi <lanurmi@iki.fi> in
<https://savannah.gnu.org/bugs/?61249>.
* gettext-tools/src/xgettext.c (main): Warn when option '--sort-output' is used.
(usage): Mark the option '--sort-output' deprecated.
* gettext-tools/doc/xgettext.texi: Likewise.
* NEWS: Mention it.
