A more App Store friendly libarchive

I've made the following changes:

- Prefixed all functions and pointer to functions with tk_ (Tim Kientzle). This should avoid any private API name conflict. The code builds without errors or warnings and I've tested the tar and untar functionality in my test project.
- Added a build scheme "arcosx" that will allow building libarcosx.a from the same source files. This is a convenience for those who want to use the project for both iOS and OSX.