Windows: short circuit background volume checks for shutdown

Windows: short circuit background volume checks for shutdown

If the service is shutting down or the machine is entering
a suspend state, short circuit the background volume checks
to permit faster shutdown.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/552
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>