Create a temporary file.
The returned file name (created by appending some random characters at the end
of PREFIX, and expanding against `temporary-file-directory' if necessary),
is guaranteed to point to a newly created empty file.
You can then use `write-region' to write new data into the file.
If DIR-FLAG is non-nil, create a new empty directory instead of a file.
If SUFFIX is non-nil, add that at the end of the file name.