Taxonomy rewrite generation when turning on permalinks, r18443 for taxonomies....

Taxonomy rewrite generation when turning on permalinks, r18443 for taxonomies. Props SergeyBiryukov. Fixes #19794.


git-svn-id: https://core.svn.wordpress.org/trunk@19930 1a063a9b-81f0-0310-95a4-ce76da25c4cd