Introduce include_selected arg for wp_dropdown_users(). Add current author...

Introduce include_selected arg for wp_dropdown_users().   Add current author to dropdown in post_author_meta_box().  fixes #16045

git-svn-id: https://core.svn.wordpress.org/trunk@17198 1a063a9b-81f0-0310-95a4-ce76da25c4cd