Advertise support for pubsub's "multi-item" feature.

XEP-0060 defines, amongst many others, the “multi-items” service discovery feature.

This feature can be advertised by a pubsub service, when clients are allowed to configure a node in such a way that it can accept more-than-one item, by:

• setting persist_items to true

• setting max_items to a value higher than 1.

Openfire supports allows for both. It can/should advertise the feature http://jabber.org/protocol/pubsub#multi-items on its pubsub services.