Windows: cm_Lookup return ambiguous filename to caller

Windows: cm_Lookup return ambiguous filename to caller

cm_Lookup() must not mask a CM_ERROR_AMBIGUOUS_FILENAME error by
converting it to CM_ERROR_BPLUS_NOMATCH.  Doing so results in the
redirector believing that the object does not exist instead of
there being a STATUS_OBJECT_NAME_COLLISION.

Change-Id: Iaa84d50271c234a84e11dd58d78ef90f5d224032
Reviewed-on: http://gerrit.openafs.org/11930
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>