use mb_disc_unix_open on darwin

This should be a start in using the new toc generation code.
use general toc code on darwin

This compiles fine, but I can't actually test it in my VM,
since Mac doesn't accept my CD drive at all.
fix first/last on Darwin

The structure doesn't give these away directly.
sessionLast is the number of the last session.
So we just calculate them.

This is now tested (with a drive) on Mac OS X.
Merge branch 'darwin_toc' into data-tracks

The changes were tested on a (physical) Mac OS X machine.