msggrep: Add new options -W and -S.

msggrep: Add new options -W and -S.

Reported at <https://savannah.gnu.org/bugs/?40489>.

* gettext-tools/src/msggrep.c (workflow_flags, sticky_flags): New variables.
(main): Initialize them. Handle the options -W and -S.
(usage): Document the -W and -S options.
(is_message_selected_no_invert): Return true if one of the specified workflow
flags or one of the specified sticky flags is set.
* gettext-tools/tests/msggrep-12: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
* gettext-tools/doc/msggrep.texi: Document the -W and -S options.
* NEWS: Mention the change.