mod_pubsub_serverinfo: Detect existence of pub/sub node Instead of blindly trying to create the pub/sub node to publish items to, a service discovery query is performed to check if node creation is required. Added various bits of warn and debug logging, to give a user better feedback if and why something is failing.

This module discards typing notifications sent to a bare host JID,
preventing error replies to be sent. These errors are harmless but can
be annoying sometimes if your client shows them prominently.