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.
Built from https://develop.svn.wordpress.org/trunk@35790


git-svn-id: http://core.svn.wordpress.org/trunk@35754 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/users.php

@@ -270,7 +270,11 @@ case 'delete':
 			<?php _e('Delete all content.'); ?></label></li>
 			<li><input type="radio" id="delete_option1" name="delete_option" value="reassign" />
 			<?php echo '<label for="delete_option1">' . __( 'Attribute all content to:' ) . '</label> ';
-			wp_dropdown_users( array( 'name' => 'reassign_user', 'exclude' => array_diff( $userids, array($current_user->ID) ) ) ); ?></li>
+			wp_dropdown_users( array(
+				'name' => 'reassign_user',
+				'exclude' => array_diff( $userids, array( $current_user->ID ) ),
+				'show' => 'display_name_with_login',
+			) ); ?></li>
 		</ul></fieldset>
 	<?php endif;
 	/**