make pluginhost a singleton

backend.php

classes/api.php

classes/feeds.php

classes/handler/public.php

classes/pluginhandler.php

classes/pluginhost.php

classes/pref/feeds.php

classes/pref/filters.php

classes/pref/labels.php

classes/pref/prefs.php

classes/pref/system.php

classes/pref/users.php

include/functions.php

include/rssfuncs.php

index.php

plugins/instances/init.php

prefs.php

public.php

update.php