Allow get_blog_option(null,...) to hit the cache for the current blog. New...

Allow get_blog_option(null,...) to hit the cache for the current blog. New return values for add_blog_option, update_blog_option, delete_blog_option. Don't set the cache in those functions if add/update/delete_option failed. see #17883.

git-svn-id: https://core.svn.wordpress.org/trunk@18662 1a063a9b-81f0-0310-95a4-ce76da25c4cd