msgmerge: New option --for-msgfmt.

msgmerge: New option --for-msgfmt.

* gettext-tools/src/msgmerge.c (for_msgfmt): New variable.
(long_options): Add option --for-msgfmt.
(main): Handle option --for-msgfmt.
(usage): Document option --for-msgfmt.
(match_domain): When --for-msgfmt is given, omit untranslated and fuzzy messages
from the result.
(merge): When --for-msgfmt is given, don't add obsolete messages to the result.
* gettext-tools/tests/msgmerge-26: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.
* gettext-tools/doc/msgmerge.texi: Mention the option --for-msgfmt.
* NEWS: Mention the change.