Windows: restrict service to 2 cpus by default

Windows: restrict service to 2 cpus by default

Performance drops off considerably when the number of processors
increases due to lock contention and the cm_SyncOp wait processing.
If the MaxCPUs registry value is not set, limit ourselves to two.
Setting MaxCPUs to zero permits use of all CPUs.

Change-Id: I4bae328ed589811b0ea2a514501a0c1aa74e8015
Reviewed-on: http://gerrit.openafs.org/6555
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>