Add cb_mktemp()

Change-Id: I5d8d282c55c3ab69eb771a5145ae407d8b268886
Reviewed-on: http://review.couchbase.org/42743
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>