mod_unified_push: Allow additional CORS headers for unified push (fixes #1985) - Request header "TTL" is mandated by the HTTP Push specification RFC 8030: https://datatracker.ietf.org/doc/html/rfc8030#section-5 - Firefox sends "Content-Encoding" with a HTTP form submission, so it is required when using the Unified Push testing tool at https://unifiedpush.org/test_wp.html (with the full URL being delivered by the UP-Example Android app) Adding both headers makes the Unified Push testing tool work from the browser. Resolves: https://issues.prosody.im/1985

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
```