Support reading PO files with '#=' lines.

Support reading PO files with '#=' lines.

* gettext-tools/src/read-catalog-abstract.c
(catalog_reader_seen_generic_comment): Treat '=' like ','.
* gettext-tools/src/read-catalog.c (default_comment_special): Allow several
calls to this method to make side effects on dcatr.
* gettext-tools/src/read-catalog-special.h: Update comment.
* gettext-tools/src/read-catalog-special.c: Likewise.
* gettext-tools/tests/msgcat-24: New file.
* gettext-tools/tests/Makefile.am (TESTS): Add it.