Windows: apply Nat Pings only to cm_rootUser connections

Windows: apply Nat Pings only to cm_rootUser connections

Use CM_UCELLFLAG_ROOTUSER flag to identify the cm_rootUser
connections and only apply Nat pings to those connections
instead of examining the security state of the connection.

Reviewed-on: http://gerrit.openafs.org/6208
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>
(cherry picked from commit 01f55bbf14210ef3e4f4c0246409af370de3a383)

Change-Id: Id26a955190aace50ca4740b910325474077941e1
Reviewed-on: http://gerrit.openafs.org/6814
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>