mod_firewall: Fix 'is_admin' internal dependency rule #1797 (thanks diane) Looks like the boolean logic was inverted here. Instead, for now, simply check if is_admin is there. It is deprecated in trunk and was briefly removed before being brought back with a 'deprecated' warning as part of the new roles and permissions work. Making this dependency conditioned on the existence of the underlying function should make it work until it actually goes away for real.

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
```