Fix plugin configuration loss

This fixes [bug 6411](https://bugs.freenetproject.org/view.php?id=6411).

    PluginManager: fix plugin configuration corruption
    
    This fixes a regression introduced by
    869f62559a06b2f72c9224a2179001dd551ea442 which corrupts plugin
    configuration by prompting getConfigLoadString() to access plugin
    startup information before startup is complete.

There's also a small cleanup. It's a mess in there.