Windows: preserve prior vlserver list on dns failure

Do not destroy the existing vlserver list if the DNS resolver query
fails.  Continue using the prior vlserver values until a DNS response
is obtained.  This will result in repeated DNS queries and a delay
if there is continued failure, but it will permit VL RPCs to continue
to be issued in the face of a DNS failure or misconfiguration.

Change-Id: Icac97c2bebdef744cc316225c1a6b1888ceb2f6e
Reviewed-on: http://gerrit.openafs.org/11457
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>