mod_sasl2_fast: Improve handling when SASL profile unexpectedly lacks CB
This fixes a traceback reported by riau, but likely does not solve the
underlying cause, whatever that is.
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
```