lib: Add ExternalBackend::getRemoteUser()

If the user is authenticated via the web server, this method should be used to retrieve the user because
it supports both reading the user from the environment or from the $_SERVER variable as fallback.

refs #11391

library/Icinga/Authentication/User/ExternalBackend.php

@@ -52,6 +52,25 @@ class ExternalBackend implements UserBackendInterface
         return $this;
     }
 
+    /**
+     * Get the remote user from environment or $_SERVER, if any
+     *
+     * @param   string  $variable   The name variable where to read the user from
+     *
+     * @return  string|null
+     */
+    public static function getRemoteUser($variable = 'REMOTE_USER')
+    {
+        $username = getenv($variable);
+        if ($username !== false) {
+            return $username;
+        }
+        if (array_key_exists($variable, $_SERVER)) {
+            return $_SERVER[$variable];
+        }
+        return null;
+    }
+
 
     /**
      * {@inheritdoc}