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.

[Install]
WantedBy=sockets.target

[Socket]
ListenStream=0.0.0.0:5222
ListenStream=5222
BindIPv6Only=ipv6-only
Accept=false
Service=prosody.service