Functions in unix.h should be private.
Use a common Unix implementation for  mb_disc_read_unportable().

This simplifies the platform implementations and avoids code duplication.
Removed unused data_tracks variable in mb_disc_load_toc().
Pass file descriptor instead of device name to mb_disc_unix_read_toc.

Avoids opening the device multiple times.
Merge remote-tracking branch 'phw/data-tracks' into data-tracks