fix (deprecated) mktemp logic and update it to match other temp functions

fix (deprecated) mktemp logic and update it to match other temp functions

the access function cannot be used to check for existence, because it
operates using real uid/gid rather than effective to determine
accessibility; this matters for the non-final path components.
instead, use stat. failure of stat is success if only the final
component is missing (ENOENT) and otherwise is failure.