Multisite: Introduce get_current_network_id()

Similar to `get_current_blog_id`, this can be used to get the ID of the `$current_site` global. If not available, it will fallback to the main network ID. In single site, this will return 1.

Props spacedmonkey, flixos90.
Fixes #33900.

Built from https://develop.svn.wordpress.org/trunk@37670


git-svn-id: http://core.svn.wordpress.org/trunk@37636 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/load.php

@@ -807,6 +807,29 @@ function get_current_blog_id() {
 	return absint($blog_id);
 }
 
+/**
+ * Retrieves the current network ID.
+ *
+ * @since 4.6.0
+ *
+ * @global WP_Network $current_site The current network.
+ *
+ * @return int The ID of the current network.
+ */
+function get_current_network_id() {
+	if ( ! is_multisite() ) {
+		return 1;
+	}
+
+	$current_site = get_current_site();
+
+	if ( ! isset( $current_site->id ) ) {
+		return get_main_network_id();
+	}
+
+	return absint( $current_site->id );
+}
+
 /**
  * Attempt an early load of translations.
  *