get ISRCs on linux, close LIB-15

On linux there is no easy ioctl command to get the ISRCs directly.
We use the SG_IO interface to issue a SCSI command.
update AUTHORS
clean-up isrc code linux
read_track_isrc: more similar to read_disc_isrc

Windows and Darwin use read_disc_isrc,
Linux now has read_track_isrc.
Keeping them similar might help when implementing multiple reads of ISRCs.

The functions still are named differently,
but I will change the name read_disc_isrc on the other platforms later on.