Rework the "ans8" server in the "digdelv" system test to support various
modes of operations using a control channel.
The supported modes are:
1. `silent` (do not respond)
2. `close` (UDP: same as `silent`; TCP: also close the connection)
3. `servfail` (always respond with `SERVFAIL`)
4. `unstable` (constantly switch between `silent` and `servfail`)
Add multiple tests to check the handling of both TCP and UDP socket
error scenarios in dig/host.
03697f1bcc