mod_pubsub_serverinfo: New module that uses pub/sub to make accessible server info This first implemetnation is laughably simple: it only adds a disco#info feature. This flags 'opt-in' for inclusion of local domain names in the data exposed by other domains (per the domain), which will allow servers to be listed in the XMPP Network Graph at https://xmppnetwork.goodbytes.im Hopefully, this bare-boned implementation acts as a stepping stone for future improvements.

Generate a DNS UPDATE order in format suitable for `nsupdate` based on
current port configuration.

Example output:

```
$ prosodyctl mod_dnsupdate -d example.com -t xmpp.example.com example.com
zone example.com
server ns1.example.com
ttl 3600
add _xmpp-client._tcp.example.com IN SRV 1 1 5222
add _xmpp-server._tcp.example.com IN SRV 1 1 5269
```