Support for unqualified domain names

[whentze]

dns.nix appears to unconditionally append a period to the name component of all records.

This rules out creating records such as

subdomain 60 IN A 1.2.3.4

i.e. ones where the domain name is considered relative to the zone origin.
This is frequently useful, for example to use the same zone file for multiple origins.

Note: I haven't looked at the code too closely, so it's possible that I'm missing something and it's already possible to do this.