Windows: prevent cm_server races

Windows: prevent cm_server races

Use interlocked operations to modified the flags field.

Close a race in cm_NewServer() which can result in multiple
cm_server objecs being created for the same addr/port/type
tuple.

Reviewed-on: http://gerrit.openafs.org/5266
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit c20010850e2b43a0b9a6d74357ea6111dbf3409e)

Change-Id: If13f23a921f598db097b391348513a3300d3b10c
Reviewed-on: http://gerrit.openafs.org/5267
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>