Join() fails to resolve unqualified DNS name

Looks like the custom TCP resolver introduced in #77 fails to resolve unqualified DNS names which IMO is a bug since all resolvers do that. It's a common pattern to use unqualified names that can be kept the same in different environments.

This affects https://github.com/prometheus/alertmanager/issues/1307

Start at Join() and inspect the custom TCP resolver introduced in issue #77. Reproduce the failure with an unqualified DNS name, then verify that Join() resolves it consistently with the expected resolver behavior; the related Alertmanager issue #1307 provides additional context.

Written by the indexing model from the issue text.