Hide used domains

Domain must be unique for mail, dns or web site, so there is no point in get used domains on domain select (with domain module enabled) and this patch get only unused domains. For subdomains it doesn't apply, so it's not used.

This patch added some code to dns module because I added domain select on merge request #105 (closed) and commit is included here because I based this branch on branch from merge request #105 (closed). I can rebase to master if you #105 (closed) is merged if it's needed.