Remove dependence on cdb_make_free function…

[ludrao]

cdb_make_finish is a tinycdb private function so it is not exported. The public function cdb_make_finish calls it anyway.
This pull request helps using lcdb with a shared library without having to modify tinycdb code.

[ludrao]

I mixed up the topic in the same master branch in my repo, so this pull request actually address 3 different issues:

• remove dependency on tinycdb internal function
• fix an actual bug that make lua-tinycdb write in global environment and prevent having several opened cdb.make isntances
• enable overwrite flag in cdb.make() API
  Let me know if you have a concern with one of this 3 changes and would like to apply them separatly.

[ludrao]

This PR fixes issue #2

[tubby1981]

Thank you folays for these commits. lua-tinycdb is working with lua5.2 with these changes.

The only error I got with example.lua is:

free(): invalid pointer
Aborted (core dumped)