Windows: Permit BPlus tree lookups within cm_ApplyDir

Windows: Permit BPlus tree lookups within cm_ApplyDir

BPlus tree lookups are much faster than searching through
the native directory format on Windows because the case sensitive
hash tables cannot be used successfully.  Permit BPlus trees
to be used except when called with cm_BPlusDirFoo as the action
function because cm_BPlusDirFoo is used to build the BPlus trees
from the native directory format.

LICENSE MIT

Change-Id: I3f6f5ba7113e206c76fafd1ec83822adc7f42f9e
Reviewed-on: http://gerrit.openafs.org/1893
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>