support *.machinename

Some users would like to have DNS names like foo.machinename.${config.dns.domains[0]} resolve to their machine, in addition to just machinename. We've gotten this request. Others might prefer that those names not resolve, maybe?

We should extend the DNS server to answer those requests, and if we're worried about unintended consequences add a column to machines that flags whether it applies.

(Other users may want machinename delegated to them; that'd be another feature, and a third value of that flag. This one definitely should not be the default.)