Improve multisite variables for WordPress multisite

[tobias]

Currently, WordPress suggest those variables to be used for a multisite setup:

$base = /YOURPATH/;
define('PATH_CURRENT_SITE', '/YOURPATH/');

To keep it simple, it should be something like:

define('PATH_CURRENT_SITE', '/YOURPATH/');
$base = PATH_CURRENT_SITE;

See ​https://github.com/WordPress/WordPress/pull/16 for the pull request that changes what I described above.

The $base global can be removed completely. See #19796. All the patches from .6.diff on remove $base.

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