Async DNS: Disable on Linux until we parse nsswitch.conf
Right now, we don't examine nsswitch.conf, but we assume it has a
standard (hosts: files dns) configuration for hostname resolution.
We should check that that's true before activating the internal
resolver, and fall back to the OS resolver if it's not.
In the meantime, however, just disable Async DNS on Linux so it doesn't
break for users with nonstandard configurations.