Enable temporary file overwrite in cdb.make() api

[ludrao]

I use the standard lua os.tmpname() to give me a temporary file name to use for the cdb.make(). For security reason os.tmpname() does create en empty name (to prevent permission stealing in extreme cases)
this make cdb.make() barely usable (I would have to remove the file before calling cdb.make())

Instead, if cdb.make would provide an option third argument that enable overwrite, it would make the api much more usable.

I send a pull request for that change.