Add 'who' arg to wp_dropdown_users() and get_users(). Add'...

Add 'who' arg to wp_dropdown_users() and get_users().  Add' hide_if_only_one_author' argument to get_users().  Query only authors (user level greater than 0) when who => author is passed.  Query only authors for author meta box and quick edit dropdowns. Props scribu. fixes #15871

git-svn-id: https://core.svn.wordpress.org/trunk@17088 1a063a9b-81f0-0310-95a4-ce76da25c4cd