Show user_login in Dashboard user dropdowns.

User dropdowns in wp-admin have traditionally shown the users' display names.
However, this causes ambiguity when users share display names. To correct this,
we now show the unique user_login in parentheses after the display name.

The new `display_name_with_login` value for the `show` parameter of
`wp_dropdown_users()` enables this functionality. The default value of `show`
has not been changed, for backward compatibility, but all instances of
`wp_dropdown_users()` in core wp-admin have been switched.

This changeset also reduces some duplicated logic when assembling a user list
when `include_selected` is true.

Props krogsgard, boonebgorges.
Fixes #31251.

git-svn-id: https://develop.svn.wordpress.org/trunk@35790 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/meta-boxes.php

@@ -742,7 +742,8 @@ function post_author_meta_box($post) {
 		'who' => 'authors',
 		'name' => 'post_author_override',
 		'selected' => empty($post->ID) ? $user_ID : $post->post_author,
-		'include_selected' => true
+		'include_selected' => true,
+		'show' => 'display_name_with_login',
 	) );
 }