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-admin/network/theme-editor.php

@@ -8,6 +8,6 @@
  */
 
 /** Load WordPress Administration Bootstrap */
-require_once( dirname( __FILE__ ) . '/admin.php' );
+require_once __DIR__ . '/admin.php';
 
-require( ABSPATH . 'wp-admin/theme-editor.php' );
+require ABSPATH . 'wp-admin/theme-editor.php';