Windows: NPAddConnection no local name and connected

The WN_ALREADY_CONNECTED error is only to be returned if a local
device name (aka drive letter) is provided in the request.  Otherwise,
the proper return status is WN_SUCCESS if an unnamed connection is
requested.

Change-Id: I1e1ca47a84e353db66e2830ec11f968a3578c4f6
Reviewed-on: http://gerrit.openafs.org/9642
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Peter Scott <pscott@kerneldrivers.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>