data-tracks improvements: generalize mb_disc_read_unportabl

This is a suggestion for the current data-tracks branch. It introduces a common Unix implementation for `mb_disc_read_unportable` again by providing additional platform sepcifc methods `mb_disc_unix_read_mcn` and `mb_disc_unix_read_isrc`.

This has the advantage of avoiding code duplication for `mb_disc_read_unportable` and simplifies the platform specific implementation. The downside is that those two additional functions must be implemented in the platform specifc code even if the respective features are unsupported. But an empty implementation will do.

I have not yet tested this on any platform but Linux, but I would like to get opinions on the general idea before doing any platform testing.