nscd: Pass TRY_AGAIN errors in the hosts cache to clients

nscd: Pass TRY_AGAIN errors in the hosts cache to clients

This is the minimal set of changes to get the upcoming test to pass.
The TTL extension logic is somewhat iffy.  There is a trade-off here:
correct operation under the DNS specification (no TTL extensions), or
reducing loading the infrastructure if TRY_AGAIN is the result of
overload from this client.

Reviewed-by: DJ Delorie <dj@redhat.com>