Windows: No rand_s on Windows 2000

Windows: No rand_s on Windows 2000

Even if the rand_s() function is supported by the compiler
it is not supported on Windows 2000 because the kernel level
functionality it requires does not exist on that platform.
Calling rand_s() on Windows 2000 will throw an exception
and terminate the service.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/464
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>