mod_sasl2, mod_sasl_bind2, mod_sasl2_sm: Bump XEP-0388 namespace
This is the namespace for the proposed update to XEP-0388. I.e. not even
experimental yet... but this is all a work in progress anyway.
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
```