Add IPv6 glue records as well

The dns_update script didn't generate IPv6 (AAAA) glue records for the name servers.

This caused http://dnscheck.pingdom.com to complain about a mismatch between the glue records reported by the parent name server and mailinabox nsd.

Here's the failing dnscheck output for reference:
> Checking glue for ns1.my.domain.tld (1.2.3.4).
> Child glue for bgwe.eu found: ns1.my.domain.tld (1.2.3.4)
> Checking glue for ns1.my.domain.tld (1234::1).
> Missing glue at child: ns1.my.domain.tld
> Checking glue for ns2.my.domain.tld (1.2.3.4).
> Child glue for bgwe.eu found: ns2.my.domain.tld (1.2.3.4)
> Checking glue for ns2.my.domain.tld (1234::1).
> Missing glue at child: ns2.my.domain.tld

I'm not very familiar with Python and DNS, please verify ;)