Removed dependency for WP default constant, by using new own constant…

[carstingaxion]

This PR removes the dependency for the WP default constant DOMAIN_CURRENT_SITE and introduce an new one called NSDU_URL, that everybody can use safely.

The most easy way would be to define( 'NSDU_URL', DOMAIN_CURRENT_SITE ); in the wp-config.php

This has 2 benefits:

1. The default constant could be used different from default, what could cause Problems, while using it for the DB updates. In example if it was set dynamically like this:

if ( isset( $_SERVER['X_FORWARDED_HOST'] ) && !empty( $_SERVER['X_FORWARDED_HOST'] ) ) {
	$hostname = $_SERVER['X_FORWARDED_HOST'];
} else {
	$hostname = $_SERVER['HTTP_HOST'];
}
define( 'DOMAIN_CURRENT_SITE', rtrim($hostname, '/') );

2. By not defining the new constant, e.g. on production sites, the Plugin would stop its work earlier and so be faster at all, while not doing its first DB hit.

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btw

Thank you very much, for your nice work and also for publishing it.

Carsten