use _mkdir on win32

Change-Id: Ie91f4a61dd2afcf4f5690840989e5634a54c6a28
Reviewed-on: http://review.couchbase.org/31206
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>