Turn is_blog_user() into a convenience wrapper around get_blogs_of_user()....

Turn is_blog_user() into a convenience wrapper around get_blogs_of_user(). Fixes is_blog_user() for blog prefixes that do not contain a blog ID. Props SergeyBiryukov. fixes #16702

git-svn-id: https://core.svn.wordpress.org/trunk@19004 1a063a9b-81f0-0310-95a4-ce76da25c4cd