Code Modernization: Replace dirname( __FILE__ ) calls with __DIR__ magic constant.

This avoids the performance overhead of the function call every time `dirname( __FILE__ )` was used instead of `__DIR__`.

This commit also includes:

* Removing unnecessary parentheses from `include`/`require` statements. These are language constructs, not function calls.
* Replacing `include` statements for several files with `require_once`, for consistency:
 * `wp-admin/admin-header.php`
 * `wp-admin/admin-footer.php`
 * `wp-includes/version.php`

Props ayeshrajans, desrosj, valentinbora, jrf, joostdevalk, netweb.
Fixes #48082.
Built from https://develop.svn.wordpress.org/trunk@47198


git-svn-id: http://core.svn.wordpress.org/trunk@46998 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/admin-bar.php

@@ -28,7 +28,7 @@ function _wp_admin_bar_init() {
 	}
 
 	/* Load the admin bar class code ready for instantiation */
-	require_once( ABSPATH . WPINC . '/class-wp-admin-bar.php' );
+	require_once ABSPATH . WPINC . '/class-wp-admin-bar.php';
 
 	/* Instantiate the admin bar */