mod_http_oauth2: Tweak method of centering the UI The percentage here was relative to the viewport width, which on some very wide screens may put the UI slightly outside of the view, requiring scrolling to see. By using a unit relative to the height of the viewport, this is avoided and should work better. But no guarantees, it's still possible to resize the browser or adjust font sizes so that the UI goes out of view.

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
```