mod_compat_roles: Fix attempt to index a nil value #1847
permissions[] is not a map with role names as keys since 817bc9873fc2
but instead a level with host names were added. This was likely an
oversight.
Refactored towards railroad.
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
```